Semiconductor device comprising oxide semiconductor film

ABSTRACT

A semiconductor device including a capacitor having an increased charge capacity without decreasing an aperture ratio is provided. The semiconductor device includes a transistor including a light-transmitting semiconductor film, a capacitor in which a dielectric film is provided between a pair of electrodes, and a pixel electrode electrically connected to the transistor. In the capacitor, a conductive film formed on the same surface as the light-transmitting semiconductor film in the transistor serves as one electrode, the pixel electrode serves as the other electrode, and a nitride insulating film and a second oxide insulating film which are provided between the light-transmitting semiconductor film and the pixel electrode serve as the a dielectric film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention disclosed in this specification and the like relates tosemiconductor devices.

2. Description of the Related Art

In recent years, flat panel displays such as liquid crystal displays(LCDs) have been widespread. In each of pixels provided in the rowdirection and the column direction in a display device such as a flatpanel display, a transistor serving as a switching element, a liquidcrystal element electrically connected to the transistor, and acapacitor connected to the liquid crystal element in parallel areprovided.

As a semiconductor material for forming a semiconductor film of thetransistor, a silicon semiconductor such as amorphous silicon orpolysilicon (polycrystalline silicon) is generally used.

Metal oxides having semiconductor characteristics (hereinafter referredto as oxide semiconductors) can be used for semiconductor films intransistors. For example, techniques for forming transistors includingzinc oxide or an In—Ga—Zn-based oxide semiconductor are disclosed (seePatent Documents 1 and 2).

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-096055

SUMMARY OF THE INVENTION

In a capacitor, a dielectric film is provided between a pair ofelectrodes at least one of which is formed, in many cases, using alight-blocking film partly serving as a gate electrode, a sourceelectrode, a drain electrode, or the like of a transistor.

As the capacitance value of a capacitor is increased, a period in whichthe alignment of liquid crystal molecules of a liquid crystal elementcan be kept constant in the state where an electric field is applied canbe made longer. When the period can be made longer in a display devicewhich displays a still image, the number of times of rewriting imagedata can be reduced, leading to a reduction in power consumption.

One of methods for increasing the charge capacity of a capacitor is toincrease the area occupied by the capacitor, specifically, to increasethe area of a region where two electrodes of the capacitor overlap witheach other. However, when the area of a light-blocking conductive filmis increased to increase the area of a region where the two electrodeoverlap with each other, the aperture ratio of a pixel is lowered andthus display quality of an image is degraded.

In view of the above problems, it is an object that is an embodiment ofthe present invention to provide a semiconductor device including acapacitor with increased charge capacity and having a high apertureratio.

An embodiment of the present invention is a semiconductor device whichincludes a transistor and a light-transmitting capacitor. Specifically,in the capacitor, a light-transmitting semiconductor film serves as oneelectrode, and a light-transmitting conductive film such as a pixelelectrode serves as the other electrode, and a light-transmittinginsulating film serves as a dielectric film. In the light-transmittinginsulating film, a first oxide insulating film, a nitride insulatingfilm, and a second oxide insulating film are stacked in this order.

An embodiment of the present invention is a semiconductor device whichincludes a transistor including a light-transmitting semiconductor film,a capacitor where a dielectric film is provided between a pair ofelectrodes, and a pixel electrode electrically connected to thetransistor. In the capacitor, a light-transmitting semiconductor filmformed on the same surface as the light-transmitting semiconductor filmin the transistor serves as one electrode, the pixel electrode serves asthe other electrode, and a light-transmitting insulating film providedover the light-transmitting semiconductor film serves as the dielectricfilm. In the light-transmitting insulating film, a first oxideinsulating film, a nitride insulating film, and a second oxideinsulating film are stacked in this order.

The first oxide insulating film is formed by a chemical vapor depositionmethod using a deposition gas containing silicon and an oxidizing gas.The nitride insulating film is formed by a chemical vapor depositionmethod or a physical vapor deposition method. The second oxideinsulating film is formed using an organosilane gas by a chemical vapordeposition method.

An oxide insulating film formed over the nitride insulating film usingan organosilane gas by a chemical vapor deposition method can improveplanarity of a surface of element portions in which the transistor andthe capacitor are formed. Further, a nitride insulating film formedbetween the transistor and the oxide insulating film formed using anorganosilane gas by a chemical vapor deposition method can preventtransfer of impurities such as carbon contained in the oxide insulatingfilms to the transistor; thus, variations in the transistor can bereduced.

The light-transmitting semiconductor film can be formed using an oxidesemiconductor. This is because an oxide semiconductor has an energy gapas wide as 3.0 eV or more and high visible-light transmissivity.

The light-transmitting capacitor can be formed using a manufacturingprocess of the transistor. One electrode of the capacitor can be formedusing a formation process of the light-transmitting semiconductor filmincluded in the transistor. The dielectric film of the capacitor can beformed using a formation process of the insulating film over thelight-transmitting semiconductor film included in the transistor. Theother electrode of the capacitor can be formed using a formation processof the pixel electrode electrically connected to the transistor. Thus,the light-transmitting semiconductor film included in the transistor andthe one electrode of the capacitor are formed using the same metalelement.

In the case where a semiconductor film formed in a step of forming thelight-transmitting semiconductor film included in the transistor is usedas the one electrode of the capacitor, the conductivity of thesemiconductor film may be increased. For example, it is preferable toadd one or more selected from boron, nitrogen, fluorine, aluminum,phosphorus, arsenic, indium, tin, antimony, and a rare gas element tothe semiconductor film. An ion implantation method, an ion dopingmethod, or the like may be employed to add the element to thesemiconductor film. Alternatively, the semiconductor film may be exposedto plasma containing the element to add the element. In that case, theone electrode of the capacitor is an n-type semiconductor film, and theconductivity of the semiconductor film is greater than or equal to 10S/cm and less than or equal to 1000 S/cm, preferably greater than orequal to 100 S/cm and less than or equal to 1000 S/cm.

With the above structure, the capacitor transmits light and thus can beformed large (covers a large area) in a region except a portion wherethe transistor in a pixel is formed. For this reason, the semiconductordevice can have charge capacity increased while improving the apertureratio. Accordingly, the semiconductor device can have excellent displayquality.

In the capacitor, the insulating film provided over thelight-transmitting semiconductor film included in the transistor is usedas the dielectric film; therefore, the dielectric film can have the samelayered structure as the insulating film. For example, in the case wherethe insulating film in which the first oxide insulating film, thenitride insulating film, and the second oxide insulating film arestacked in this order serves as the insulating film provided over thelight-transmitting semiconductor film included in the transistor, theinsulating film in which the first oxide insulating film, the nitrideinsulating film, and the second oxide insulating film are stacked inthis order can serve as the dielectric film of the capacitor.

In the case where in the capacitor, the first oxide insulating film, thenitride insulating film, and the second oxide insulating film which arestacked in this order serve as the insulating film provided over thelight-transmitting semiconductor film included in the transistor, only aportion of the first oxide insulating film in which the capacitor isformed is removed after the oxide insulating film is formed, whereby thenitride insulating film and the second oxide insulating film can serveas the dielectric film of the capacitor. In other words, the nitrideinsulating film is in contact with the semiconductor film serving as thepair of electrodes of the capacitor. Since the semiconductor filmincludes an oxide semiconductor, a defect state (an interface state) isformed at the interface between the nitride insulating film and thesemiconductor film when the nitride insulating film and thesemiconductor film are in contact with each other. In addition oralternatively, when the nitride insulating film is formed by a plasmaCVD method or a sputtering method, the semiconductor film is exposed toplasma, so that oxygen vacancies are generated. Further, nitrogen and/orhydrogen contained in the nitride insulating film move(s) to thesemiconductor film. Hydrogen contained in the nitride insulating filmenters the defect state or oxygen vacancies, whereby electrons servingas carriers can be generated. As a result, the semiconductor film has ahigher conductivity and becomes n-type, and thus has conductivity. Thatis, a metal oxide film having characteristics of a conductor can beformed. Further, the thickness of the dielectric film can be reduced;therefore, an increase in the charge capacity of the capacitor can beachieved.

When the nitride insulating film is in contact with the semiconductorfilm in the capacitor as described above, a step of adding an elementwhich increases the conductivity to the semiconductor film by an ionimplantation method, an ion doping method, or the like can be skipped;therefore, the yield of the semiconductor device can be increased andthe manufacturing cost thereof can be reduced.

In the case where the light-transmitting semiconductor film included inthe transistor is formed using an oxide semiconductor and the insulatingfilm in which the first oxide insulating film, the nitride insulatingfilm, and the second oxide insulating film are stacked in this orderserves as the insulating film provided over the light-transmittingsemiconductor film, it is preferable that the oxide insulating film beunlikely to transmit nitrogen and hydrogen, that is, that the oxideinsulating film have a barrier property to nitrogen.

With the above structure, one or both of nitrogen and hydrogen can beprevented from diffusing into the light-transmitting semiconductor filmincluded in the transistor, so that variations in the electricalcharacteristics of the transistor can be suppressed.

In the above, the semiconductor device that is an embodiment of thepresent invention includes a scan line including a gate electrode of thetransistor and a capacitor line extending in the direction parallel tothe scan line on the same surface as the scan line. One electrode(semiconductor film) of the capacitor is electrically connected to thecapacitor line through a conductive film which can be formed at the sametime as formation of source and drain electrodes of the transistor.Alternatively, the capacitor line is formed using a semiconductor filmincluded in the capacitor.

The capacitor line does not necessarily extend in the direction parallelto a scan line on the same surface as the scan line. The capacitor linemay extend in the direction parallel to a scan line including the sourceelectrode or the drain electrode of the transistor on the same surfaceas the signal line, and may be electrically connected to one electrode(semiconductor film) of the capacitor.

The capacitor line may be connected to each of capacitors included in aplurality of adjacent pixels. In this case, the capacitor line may beprovided between the adjacent pixels.

Further, in the case where the semiconductor film formed at the sametime as the light-transmitting semiconductor film included in thetransistor serves as one electrode of the capacitor, the conductive filmin contact with the semiconductor film and the capacitor line may beprovided in contact with an end portion of the semiconductor film and,for example, can be provided in contact with the semiconductor filmalong the outer periphery thereof. With such a structure, theconductivity of the semiconductor film can be increased.

A manufacturing method of a semiconductor device that is an embodimentof the present invention is an embodiment of the present invention.

According to an embodiment of the present invention, a semiconductordevice including a capacitor whose charge capacity is increased whileimproving the aperture ratio can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a semiconductor device that is an embodiment of thepresent invention and FIGS. 1B and 1C are circuit diagrams eachillustrating a pixel.

FIG. 2 is a top view illustrating a semiconductor device that is anembodiment of the present invention.

FIG. 3 is a cross-sectional view illustrating a semiconductor devicethat is an embodiment of the present invention.

FIGS. 4A and 4B are cross-sectional views illustrating a manufacturingmethod of a semiconductor device that is an embodiment of the presentinvention.

FIGS. 5A and 5B are cross-sectional views illustrating a manufacturingmethod of a semiconductor device that is an embodiment of the presentinvention.

FIG. 6 is a top view illustrating a semiconductor device that is anembodiment of the present invention.

FIG. 7 is a top view illustrating a semiconductor device that is anembodiment of the present invention.

FIG. 8 is a top view illustrating a semiconductor device that is anembodiment of the present invention.

FIG. 9 is a top view illustrating a semiconductor device that is anembodiment of the present invention.

FIG. 10 is a cross-sectional view illustrating a semiconductor devicethat is an embodiment of the present invention.

FIGS. 11A and 11B are cross-sectional views illustrating a manufacturingmethod of a semiconductor device that is an embodiment of the presentinvention.

FIGS. 12A and 12B are cross-sectional views illustrating a manufacturingmethod of a semiconductor device that is an embodiment of the presentinvention.

FIG. 13 is a cross-sectional view illustrating a manufacturing method ofa semiconductor device that is an embodiment of the present invention.

FIG. 14 is a top view illustrating a semiconductor device that is anembodiment of the present invention.

FIG. 15 is a cross-sectional view illustrating a semiconductor devicethat is an embodiment of the present invention.

FIGS. 16A and 16B are cross-sectional views illustrating a manufacturingmethod of a semiconductor device that is an embodiment of the presentinvention.

FIGS. 17A and 17B are cross-sectional views illustrating a manufacturingmethod of a semiconductor device that is an embodiment of the presentinvention.

FIG. 18 is a cross-sectional view of a semiconductor device that is anembodiment of the present invention.

FIGS. 19A to 19C are top views illustrating a semiconductor device thatis an embodiment of the present invention.

FIG. 20 is a cross-sectional view of a semiconductor device that is anembodiment of the present invention.

FIGS. 21A to 21C are cross-sectional views and a top view illustrating asemiconductor device that is an embodiment of the present invention.

FIGS. 22A to 22C illustrate electronic devices in each of which asemiconductor device that is an embodiment of the present invention isused.

FIGS. 23A to 23C illustrate an electronic device in which asemiconductor device that is an embodiment of the present invention isused.

FIGS. 24A to 24D each illustrate a structure of a sample.

FIG. 25 is a graph showing sheet resistance.

FIGS. 26A and 26B show results of SIMS measurement.

FIGS. 27A to 27C are graphs showing results of ESR measurement.

FIG. 28 is a graph showing results of ESR measurement.

FIG. 29 is a graph showing sheet resistance.

FIG. 30 is a graph showing sheet resistance.

FIGS. 31A to 31D are diagrams describing bulk models of InGaZnO₄.

FIGS. 32A and 32B are a graph and a diagram describing formation energyand a thermodynamic transition level of VoH, respectively.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments and examples of the present invention are described below indetail with reference to the accompanying drawings. However, the presentinvention is not limited to the following description and it is easilyunderstood by those skilled in the art that the mode and details can bevariously changed. In addition, the present invention should not beconstrued as being limited to the description in the followingembodiments and examples.

Note that in structures of the present invention described below, thesame portions or portions having similar functions are denoted by thesame reference numerals in different drawings, and description thereofis not repeated. Further, the same hatching pattern is applied toportions having similar functions, and in some cases the portions arenot especially denoted by reference numerals.

Note that in each drawing described in this specification, the size, thefilm thickness, or the region of each component is in some casesexaggerated for clarity. Therefore, the embodiments and the examples ofthe present invention are not limited to such scales.

Note that the ordinal numbers such as “first” and “second in thisspecification and the like are used for convenience and do not indicatethe order of steps or the stacking order of layers. In addition, theordinal numbers in this specification and the like do not indicateparticular names which specify the present invention.

Note that a voltage refers to a difference between potentials of twopoints, and a potential refers to electrostatic energy (electricpotential energy) of a unit charge at a given point in an electrostaticfield. In general, a difference between a potential of one point and areference potential (e.g., a ground potential) is merely called apotential or a voltage, and a potential and a voltage are used in manycases as synonymous words. Thus, in this specification, a potential maybe rephrased as a voltage and a voltage may be rephrased as a potentialunless otherwise specified.

In this specification, in the case where etching treatment is performedafter a photolithography process, a mask formed in the photolithographyprocess is removed after the etching treatment.

Embodiment 1

In this embodiment, a semiconductor device that is an embodiment of thepresent invention is described with reference to drawings. Note that inthis embodiment, a semiconductor device that is an embodiment of thepresent invention is described taking a liquid crystal display device asan example.

<Structure of Semiconductor Device>

FIG. 1A illustrates an example of a semiconductor device. Thesemiconductor device in FIG. 1A includes a pixel portion 100, a scanline driver circuit 104, a signal line driver circuit 106, m scan lines107 which are arranged in parallel or substantially in parallel andwhose potentials are controlled by the scan line driver circuit 104, andn signal lines 109 which are arranged in parallel or substantially inparallel and whose potentials are controlled by the signal line drivercircuit 106. Further, the pixel portion 100 includes a plurality ofpixels 101 arranged in a matrix. Furthermore, capacitor lines 115arranged in parallel or substantially in parallel are provided along thescan lines 107. Note that the capacitor lines 115 may be arranged inparallel or substantially in parallel along the signal lines 109.

Each scan line 107 is electrically connected to the n pixels 101 in thecorresponding row among the pixels 101 arranged in m rows and n columnsin the pixel portion 100. Each signal line 109 is electrically connectedto the m pixels 101 in the corresponding column among the pixels 101arranged in m rows and n columns. Note that m and n are each an integerof 1 or more. Each capacitor line 115 is electrically connected to the npixels 101 in the corresponding row among the pixels 101 arranged in mrows and n columns. Note that in the case where the capacitor lines 115are arranged in parallel or substantially in parallel along the signallines 109, each capacitor line 115 is electrically connected to the mpixels 101 in the corresponding column among the pixels 101 arranged inm rows and n columns.

FIG. 1B is an example of a circuit diagram of the pixel 101 included inthe semiconductor device illustrated in FIG. 1A. The pixel 101 in FIG.1B includes a transistor 103 which is electrically connected to the scanline 107 and the signal line 109, a capacitor 105 one electrode of whichis electrically connected to a drain electrode of the transistor 103 andthe other electrode of which is electrically connected to the capacitorline 115 which supplies a constant potential, and a liquid crystalelement 108. A pixel electrode of the liquid crystal element 108 iselectrically connected to the drain electrode of the transistor 103 andthe one electrode of the capacitor 105, and an electrode (counterelectrode) facing the pixel electrode is electrically connected to awiring which supplies a counter potential.

The liquid crystal element 108 is an element which controls transmissionof light by an optical modulation action of liquid crystal which issandwiched between a substrate provided with the transistor 103 and thepixel electrode and a substrate provided with the counter electrode. Theoptical modulation action of liquid crystal is controlled by an electricfield applied to the liquid crystal (including a vertical electric fieldand a diagonal electric field). Note that in the case where a counterelectrode (also referred to as a common electrode) is provided over asubstrate where a pixel electrode is provided, an electric field appliedto liquid crystal is a transverse electric field.

Next, a specific example of the pixel 101 of the liquid crystal displaydevice is described. FIG. 2 is a top view of the pixel 101. Note that inFIG. 2, the counter electrode and the liquid crystal element areomitted.

In FIG. 2, the scan line 107 is provided so as to extend in thedirection perpendicular or substantially perpendicular to the signalline 109 (in the horizontal direction in the drawing). The signal line109 is provided so as to extend in the direction perpendicular orsubstantially perpendicular to the scan line 107 (in the verticaldirection in the drawing). The capacitor line 115 is provided so as toextend in the direction parallel to the scan line 107. The scan line 107and the capacitor line 115 are electrically connected to the scan linedriver circuit 104 (see FIG. 1A), and the signal line 109 iselectrically connected to the signal line driver circuit 106 (see FIG.1A).

The transistor 103 is provided in a region where the scan line 107 andthe signal line 109 cross each other. The transistor 103 includes atleast a semiconductor film 111 including a channel formation region, agate electrode, a gate insulating film (not illustrated in FIG. 2), asource electrode, and a drain electrode. A portion of the scan line 107which overlaps with the semiconductor film 111 functions as the gateelectrode of the transistor 103. A portion of the signal line 109 whichoverlaps with the semiconductor film 111 functions as the sourceelectrode of the transistor 103. A portion of a conductive film 113which overlaps with the semiconductor film 111 functions as the drainelectrode of the transistor 103. Thus, the gate electrode, the sourceelectrode, and the drain electrode may be referred to as the scan line107, the signal line 109, and the conductive film 113, respectively.Further, in FIG. 2, an edge of the scan line 107 is on the outer side ofan edge of the semiconductor film when seen from above. Thus, the scanline 107 functions as a light-blocking film for blocking light from alight source such as a backlight. For this reason, the semiconductorfilm 111 included in the transistor is not irradiated with light, sothat variations in the electrical characteristics of the transistor canbe suppressed.

Further, an oxide semiconductor processed under appropriate conditionscan significantly reduce the off-state current of a transistor;therefore, such an oxide semiconductor is used for the semiconductorfilm 111 in an embodiment of the present invention. Thus, powerconsumption of a semiconductor device can be reduced.

The conductive film 113 is electrically connected to a pixel electrode121 formed using a light-transmitting conductive film, through anopening 117. In FIG. 2, the hatch pattern of the pixel electrode 121 isnot shown.

The capacitor 105 is provided in a region which is in the pixel 101 andsurrounded by capacitor lines 115 and signal lines 109. The capacitor105 is electrically connected to the capacitor line 115 through aconductive film 125 provided in and over an opening 123. The capacitor105 includes a semiconductor film 119 including an oxide semiconductor,the pixel electrode 121, and an insulating film (not illustrated in FIG.2) which is formed as a dielectric film over the transistor 103. Thesemiconductor film 119, the pixel electrode 121, and the dielectric filmtransmit light; accordingly, the capacitor 105 transmits light.

Thanks to the light-transmitting property of the semiconductor film 119,the capacitor 105 can be formed large (covers a large area) in the pixel101. Thus, a semiconductor device having increased charge capacity whileimproving the aperture ratio, to typically 55% or more, preferably 60%or more can be obtained. For example, in a semiconductor device with ahigh resolution such as a liquid crystal display device, the area of apixel is small and thus the area of a capacitor is also small. For thisreason, the charge capacity of the capacitor is small in a semiconductordevice with a high resolution. However, since the capacitor 105 of thisembodiment transmits light, when it is provided in a pixel, enoughcharge capacity can be obtained in the pixel and the aperture ratio canbe improved. Typically, the capacitor 105 can be favorably used in ahigh-resolution semiconductor device with a pixel density of 200 ppi ormore, or furthermore, 300 ppi or more. Further, according to anembodiment of the present invention, the aperture ratio can be improvedeven in a display device with a high resolution, which makes it possibleto use light from a light source such as a backlight efficiently, sothat power consumption of the display device can be reduced.

Here, the characteristics of a transistor including an oxidesemiconductor are described. Note that the transistor including an oxidesemiconductor is an n-channel transistor. Further, carriers might begenerated due to oxygen vacancies in the oxide semiconductor, whichmight degrade the electrical characteristics and reliability of thetransistor. For example, in some cases, the threshold voltage of thetransistor is shifted in the negative direction, and drain current flowswhen the gate voltage is 0 V. A transistor in which drain current flowswhen the gate voltage is 0 V is referred to as a normally-on transistor.A transistor in which substantially no drain current flows when the gatevoltage is 0 V is referred to as a normally-off transistor.

In view of the above, it is preferable that defects in the semiconductorfilm 111, typically oxygen vacancies, be reduced as much as possiblewhen an oxide semiconductor is used for the semiconductor film 111. Forexample, it is preferable that the spin density of the oxidesemiconductor film (the density of defects in the semiconductor film111) at a g-value of 1.93 in electron spin resonance spectroscopy inwhich a magnetic field is applied in parallel to the film surface bereduced to lower than or equal to the lower detection limit ofmeasurement equipment. When the defects typified by oxygen vacancies inthe semiconductor film 111 are reduced as much as possible, thetransistor 103 can be prevented from being normally on, leading toimprovements in the electrical characteristics and reliability of asemiconductor device.

The shift of the threshold voltage of a transistor in the negativedirection is caused by hydrogen (including a hydrogen compound such aswater) contained in an oxide semiconductor in some cases as well as byoxygen vacancies. Hydrogen contained in the oxide semiconductor isreacted with oxygen bonded to a metal atom to be water, and in addition,vacancies (also referred to as oxygen vacancies) are formed in a latticefrom which oxygen is released (or a portion from which oxygen isremoved). In addition, the reaction of part of hydrogen and oxygencauses generation of electrons serving as carriers. Thus, a transistorincluding an oxide semiconductor which contains hydrogen is likely to benormally on.

In view of the above, when an oxide semiconductor is used for thesemiconductor film 111, it is preferable that hydrogen in thesemiconductor film as the semiconductor film 111 be reduced as much aspossible. Specifically, the concentration of hydrogen in thesemiconductor film 111, which is measured by secondary ion massspectrometry (SIMS), is set to lower than 5×10¹⁸ atoms/cm³, preferablylower than or equal to 1×10¹⁸ atoms/cm³, more preferably lower than orequal to 5×10¹⁷ atoms/cm³, still more preferably lower than or equal to1×10¹⁶ atoms/cm³.

The concentration of alkali metals or alkaline earth metals in thesemiconductor film 111, which is measured by secondary ion massspectrometry (SIMS), is set to lower than or equal to 1×10¹⁸ atoms/cm³,preferably lower than or equal to 2×10¹⁶ atoms/cm³. This is because analkali metal and an alkaline earth metal might generate carriers whenbonded to an oxide semiconductor, in which case the off-state current ofthe transistor 103 might be increased.

Further, when nitrogen is contained in the semiconductor film 111,electrons serving as carriers are generated and the carrier densityincreases, so that the semiconductor film easily becomes n-type. Thus, atransistor including an oxide semiconductor which contains nitrogen islikely to be normally on. For this reason, nitrogen in the semiconductorfilm 111 is preferably reduced as much as possible; the concentration ofnitrogen is preferably set to, for example, lower than or equal to5×10¹⁸ atoms/cm³.

When such an oxide semiconductor film highly purified by reducingimpurities (such as hydrogen, nitrogen, an alkali metal, and an alkalineearth metal) as much as possible is used as the semiconductor film 111,the transistor 103 can be prevented from being normally on, so that theoff-state current of the transistor 103 can be significantly reduced.Accordingly, a semiconductor device having favorable electricalcharacteristics can be manufactured. Further, a semiconductor devicewith improved reliability can be manufactured.

Various experiments can prove the low off-state current of a transistorincluding a highly purified oxide semiconductor film. For example, evenwhen an element has a channel width (W) of 1×10⁶ μm and a channel length(L) of 10 μm, off-state current can be less than or equal to themeasurement limit of a semiconductor parameter analyzer, i.e., less thanor equal to 1×10⁻¹³ A, at a voltage (drain voltage) between the sourceelectrode and the drain electrode of from 1 V to 10 V. In this case, itcan be found that the off-state current corresponding to a valueobtained by dividing the off-state current by the channel width of thetransistor is 100 zA/μm or lower. Further, the off-state current wasmeasured with the use of a circuit in which a capacitor and a transistorare connected to each other and charge that flows in or out from thecapacitor is controlled by the transistor. In the measurement, a highlypurified oxide semiconductor film has been used for a channel formationregion of the transistor, and the off-state current of the transistorhas been measured from a change in the amount of charge of the capacitorper unit time. As a result, it is found that in the case where thevoltage between the source electrode and the drain electrode of thetransistor is 3 V, lower off-state current of several tens ofyoctoamperes per micrometer (yA/μm) can be obtained. Thus, thetransistor including the highly purified oxide semiconductor film has asignificantly low off-state current.

Next, FIG. 3 illustrates a cross-sectional view taken alongdashed-dotted line A1-A2, dashed-dotted line B1-B2, and dashed-dottedline C1-C2 in FIG. 2. Further, FIG. 3 illustrates a cross-sectional viewof the scan line driver circuit 104 taken along dashed-dotted lineD1-D2, though the top view of the scan line driver circuit 104 isomitted. The cross-sectional view of the transistor provided in the scanline driver circuit 104 is illustrated here, and the transistor can beprovided in the signal line driver circuit 106.

First, structures of the pixel 101 taken along dashed-dotted line A1-A2,dashed-dotted line B1-B2, and dashed-dotted line C1-C2 are described.The scan line 107 including a gate electrode of the transistor 103 andthe capacitor line 115 on the same surface as the scan line 107 areprovided over a substrate 102. A gate insulating film 127 is providedover the scan line 107 and the capacitor line 115. The semiconductorfilm 111 is provided over a portion of the gate insulating film 127which overlaps with the scan line 107, and the semiconductor film 119 isprovided over the gate insulating film 127. The signal line 109including a source electrode of the transistor 103 and the conductivefilm 113 including a drain electrode of the transistor 103 are providedover the semiconductor film 111 and the gate insulating film 127. Theopening 123 reaching the capacitor line 115 is formed in the gateinsulating film 127, and the conductive film 125 is provided in and overthe opening 123 and over the gate insulating film 127 and thesemiconductor film 119. An insulating film 129, an insulating film 131,an insulating film 132, and an insulating film 137 functioning asprotective insulating films of the transistor 103 are provided over thegate insulating film 127, the signal line 109, the semiconductor film111, the conductive films 113 and 125, and the semiconductor film 119.The opening 117 reaching the conductive film 113 (see FIG. 2) is formedin the insulating films 129, 131, 132, and 137, and the pixel electrode121 is provided in the opening 117 (see FIG. 2) and over the insulatingfilm 137.

In the capacitor 105 described in this embodiment, the semiconductorfilm 119 formed in the same process as the semiconductor film 111 servesas one of a pair of electrodes, the pixel electrode 121 serves as theother of the pair of electrodes, and the insulating films 129, 131, 132,and 137 serve as a dielectric film provided between the pair ofelectrodes.

Next, the structure of the transistor provided in the scan line drivercircuit 104 is described. A gate electrode 627 of a transistor 623 isprovided over the substrate 102. The gate insulating film 127 isprovided over the gate electrode 627. A semiconductor film 631 isprovided over a region of the gate insulating film 127 which overlapswith the gate electrode 627. A source electrode 629 and a drainelectrode 633 of the transistor 623 are provided over the semiconductorfilm 631 and the gate insulating film 127. Further, the insulating films129, 131, 132, and 137 serving as protective insulating films of thetransistor 623 are provided over the gate insulating film 127, thesource electrode 629, the semiconductor film 631, and the drainelectrode 633. A conductive film 641 is provided over the insulatingfilm 137.

Note that a base insulating film may be provided between the substrate102, and the scan line 107, the capacitor line 115, the gate electrode627, and the gate insulating film 127.

In the transistor 623, the conductive film 641 overlapping with the gateelectrode 627 with the semiconductor film 631 interposed therebetween isprovided, whereby variations in gate voltage at which an on-currentrises at different drain voltages can be reduced. Further, a currentflowing between the source electrode 629 and the drain electrode 633 ata surface of the semiconductor film 631 facing the conductive film 641can be controlled and thus variations in electrical characteristicsbetween different transistors can be reduced. Further, when theconductive film 641 is provided, an influence of a change in ambientelectric field on the semiconductor film 631 can be reduced, leading toan improvement in reliability of the transistor. Further, when thepotential of the conductive film 641 is the same or substantially thesame as the minimum potential (Vss; for example, the potential of thesource electrode 629 in the case where the potential of the sourceelectrode 629 is a reference potential), variations in threshold voltageof the transistor can be reduced and the reliability of the transistorcan be improved.

The insulating films 129 and 131 can be formed to have a single-layerstructure or a layered structure using, for example, any of oxideinsulating materials such as silicon oxide, silicon oxynitride, aluminumoxide, hafnium oxide, gallium oxide, and a Ga—Zn-based metal oxide.

The insulating film 129 can have a thickness of greater than or equal to5 nm and less than or equal to 150 nm, preferably greater than or equalto 5 nm and less than or equal to 50 nm, more preferably greater than orequal to 10 nm and less than or equal to 30 nm. The insulating film 131can have a thickness of greater than or equal to 30 nm and less than orequal to 500 nm, preferably greater than or equal to 150 nm and lessthan or equal to 400 nm.

Further, the insulating film 132 can be formed using a nitrideinsulating material such as silicon nitride oxide, silicon nitride,aluminum nitride, aluminum nitride oxide, and the like to have asingle-layer structure or a stacked structure.

As the insulating film 132, a nitride insulating film where the hydrogencontent is low may be provided. The nitride insulating film is asfollows, for example: the number of hydrogen molecules released from thenitride insulating film is less than 5.0×10²¹ molecules/cm³, preferablyless than 3.0×10²¹ molecules/cm³, more preferably less than 1.0×10²¹molecules/cm³ when measured by TDS spectroscopy.

The insulating film 132 has a thickness large enough to prevent entry ofimpurities such as hydrogen and water from the outside. For example, thethickness can become greater than or equal to 50 nm and less than orequal to 200 nm, preferably greater than or equal to 50 nm and less thanor equal to 150 nm, and further preferably greater than or equal to 50nm and less than or equal to 100 nm.

An oxide insulating film formed by a chemical vapor deposition (CVD)method using an organosilane gas, typically a silicon oxide film, can beused as the insulating film 137.

As the insulating film 137, a silicon oxide film formed by a CVD methodusing an organosilane gas is provided. The silicon oxide film can beformed to a thickness of 300 nm to 600 nm inclusive. As the organosilanegas, any of the following silicon-containing compound can be used:tetraethyl orthosilicate (TEOS) (chemical formula: Si(OC₂H₅)₄);tetramethylsilane (TMS) (chemical formula: Si(CH₃)₄);tetramethylcyclotetrasiloxane (TMCTS); octamethylcyclotetrasiloxane(OMCTS); hexamethyldisilazane (HMDS); triethoxysilane (SiH(OC₂H₅)₃);trisdimethylaminosilane (SiH(N(CH₃)₂)₃); or the like.

When the insulating film 137 is formed using a silicon oxide film formedby a CVD method using an organosilane gas, the planarity of a surface ofan element portion over the substrate 102 can be improved. As a result,without a planarization film formed using an organic resin, alignmentdisorder of the liquid crystal molecules can be reduced, light leakagecan be reduced, and contrast can be enhanced. Here, the element portionincludes the transistors formed over the substrate 102, protectiveinsulating films provided over the transistors (the insulating films129, 131, 132, and 137), the capacitor, and the conductive film formedover the protective insulating films. The insulating film 129 is notnecessarily provided.

The insulating film 132 which is a nitride insulating film is providedbetween the insulating film 137 and the transistors 103 and 623, wherebyimpurities such as carbon contained in the silicon oxide film areblocked by the insulating film 132, so that transfer of impurities tothe semiconductor film 111 of the transistor 103 and the semiconductorfilm 631 of the transistor 623 is reduced; thus, variations inelectrical characteristics of the transistors can be reduced.

Further, an oxide insulating film in which the oxygen content is higherthan that in the stoichiometric composition is preferably used as one orboth of the insulating film 129 and the insulating film 131. In thismanner, oxygen can be prevented from being released from thesemiconductor films 111 and 631, and oxygen contained in an oxygenexcess region can be transferred to the semiconductor films 111 and 631.The oxygen transferred to the semiconductor films 111 and 631 can reduceoxygen vacancies in the oxide semiconductor included in thesemiconductor films 111 and 631. For example, when an oxide insulatingfilm having the following feature is used, oxygen vacancies in thesemiconductor films 111 and 631 can be reduced. The feature of the oxideinsulating film is that the number of oxygen molecules released from theoxide insulating film is greater than or equal to 1.0×10¹⁸ molecules/cm³when measured by thermal desorption spectroscopy (hereinafter referredto as TDS spectroscopy). Note that an oxide insulating film partlyincluding a region in which the oxygen content is higher than that inthe stoichiometric composition (oxygen excess region) may be used as oneor both of the insulating film 129 and the insulating film 131. Whensuch an oxygen excess region is present in a region overlapping with atleast the semiconductor films 111 and 631, oxygen is prevented frombeing released from the semiconductor films 111 and 631 and the oxygencontained in the oxygen excess region can enter the semiconductor films111 and 631 to reduce oxygen vacancies.

In the case where the insulating film 131 is an oxide insulating film inwhich the oxygen content is higher than that in the stoichiometriccomposition, the insulating film 129 is preferably an oxide insulatingfilm that oxygen can permeate. Oxygen which enters the insulating film129 from the outside does not completely go through the insulating film129 to be released into the semiconductor films 111 and 631 and partthereof remains in the insulating film 129. Further, there is oxygenwhich is contained in the insulating film 129 from the start and isreleased from the insulating film 129 to the outside. Thus, theinsulating film 129 preferably has a high coefficient of diffusion ofoxygen.

Since the insulating film 129 is in contact with the semiconductor films111 and 631 formed using an oxide semiconductor, the insulating film 129is preferably an oxide insulating film that oxygen can permeate andwhich has a low interface state density with the semiconductor films 111and 631. For example, the insulating film 129 is preferably an oxideinsulating film having a lower defect density than the insulating film131. Specifically, the spin density of the oxide insulating film at ag-value of 2.001 (E′-center) obtained by electron spin resonance is3.0×10¹⁷ spins/cm³ or lower, preferably 5.0×10¹⁶ spins/cm³ or lower. Thespin density at a g-value of 2.001 measured by electron spin resonancespectroscopy corresponds to the number of dangling bonds in theinsulating film 129.

Further, an insulating film having a barrier property against nitrogenis preferably used as one or both of the insulating film 129 and theinsulating film 131. For example, a dense oxide insulating film has abarrier property against nitrogen. Specifically, an oxide insulatingfilm which can be etched by 0.5 wt % of fluoric acid at a rate of lessthan or equal to 10 nm per minute when the temperature is 25° C. ispreferably used.

In the case where an oxide insulating film containing nitrogen, such asa silicon oxynitride film or a silicon nitride oxide film, is used asone or both of the insulating film 129 and the insulating film 131, thenitrogen concentration measured by SIMS is greater than or equal to thelower limit of measurement by SIMS and less than 3×10²⁰ atoms/cm³,preferably greater than or equal to 1×10¹⁸ atoms/cm³ and less than orequal to 1×10²⁰ atoms/cm³. In that case, the amount of nitrogen whichenters the semiconductor films 111 and 631 included in the transistor103 can be reduced and the number of defects in the nitrogen-containingoxide insulating film itself can be reduced.

Further, the insulating films 129, 131, 132, and 137 are providedbetween the signal line 109 and the pixel electrode 121 in the pixel, sothat the distance between the signal line 109 and the pixel electrode121 is increased. Accordingly, parasitic capacitance can be lowered andthus display unevenness and an increase in power consumption can bereduced due to delay of a signal voltage applied to the signal line 109.In the transistor 623 included in the scan line driver circuit, thedistance between the semiconductor film 631 and the conductive film 641is increased; thus, an electric field of the conductive film 641affecting the semiconductor film 631 is relaxed, so that variations inelectrical characteristics of the transistor 623 can be reduced.Accordingly, the semiconductor device can have excellent displayquality.

The details of the components of the above structure are describedbelow.

There is no particular limitation on the property of a material and thelike of the substrate 102 as long as the material has heat resistanceenough to withstand at least heat treatment performed in a manufacturingprocess of the semiconductor device. Examples of the substrate are aglass substrate, a ceramic substrate, and a plastic substrate, and asthe glass substrate, an alkali-free glass substrate such as a bariumborosilicate glass substrate, an aluminoborosilicate glass substrate, oran aluminosilicate glass substrate is preferably used. Alternatively, anon-light-transmitting substrate such as a stainless alloy substrate maybe used, in which case a surface of the substrate is preferably providedwith an insulating film. As the substrate 102, any of the following mayalternatively be used: a quartz substrate, a sapphire substrate, asingle crystal semiconductor substrate, a polycrystalline semiconductorsubstrate, a compound semiconductor substrate, a silicon on insulator(SOI) substrate, and the like.

The scan line 107, the capacitor line 115, and the gate electrode 627through which a large amount of current flows are preferably formed tohave a single-layer structure or a stacked-layer structure using a metalfilm, typically any of metal materials such as molybdenum (Mo), titanium(Ti), tungsten (W), tantalum (Ta), aluminum (Al), copper (Cu), chromium(Cr), neodymium (Nd), or scandium (Sc), or an alloy material whichcontains any of these materials as its main component.

Examples of the scan line 107, the capacitor line 115, and the gateelectrode 627 are a single-layer structure using aluminum containingsilicon, a two-layer structure in which titanium is stacked overaluminum, a two-layer structure in which titanium is stacked over atitanium nitride, a two-layer structure in which tungsten is stackedover a titanium nitride, a two-layer structure in which tungsten isstacked over a tantalum nitride, a two-layer structure in which copperis stacked over a copper-magnesium-aluminum alloy, and a three-layerstructure in which titanium nitride, copper, and tungsten are stacked inthis order.

As a material of the scan line 107, the capacitor line 115, and the gateelectrode 627, a light-transmitting conductive material that can be usedfor the pixel electrode 121 can be used.

Further, as the material of the scan line 107, the capacitor line 115,and the gate electrode 627, a metal oxide containing nitrogen,specifically, an In—Ga—Zn-based oxide containing nitrogen, anIn—Sn-based oxide containing nitrogen, an In—Ga-based oxide containingnitrogen, an In—Zn-based oxide containing nitrogen, a Sn-based oxidecontaining nitrogen, an In-based oxide containing nitrogen, or a metalnitride (InN, SnN, or the like) film can be used. These materials eachhave a work function higher than or equal to 5 eV (electron volts). Inthe case where the semiconductor film 111 of the transistor 103 isformed using an oxide semiconductor, the use of a metal oxide containingnitrogen for the scan line 107 (the gate electrode of the transistor103) allows the threshold voltage of the transistor 103 to be shifted inthe positive direction, i.e. the transistor can have normally-offcharacteristics. For example, in the case where an In—Ga—Zn-based oxidecontaining nitrogen is used, an In—Ga—Zn-based oxide having at least ahigher nitrogen concentration than the semiconductor film 111,specifically an In—Ga—Zn-based oxide having a nitrogen concentration of7 at. % or higher can be used.

It is preferable to use aluminum or copper which is a low-resistantmaterial for the scan line 107, the capacitor line 115, and the gateelectrode 627. When aluminum or copper is used, signal delay is reduced,so that the display quality can be improved. Note that aluminum has lowheat resistance; therefore, defects due to a hillock, a whisker, ormigration tend to be caused. In order to prevent migration of aluminum,a stacked-layer structure including aluminum and a metal material havinga higher melting point than aluminum, such as molybdenum, titanium, ortungsten, is preferably used. Also when copper is used, in order toprevent a defect due to migration and diffusion of copper elements, astacked-layer structure including copper and a metal material having ahigher melting point than copper, such as molybdenum, titanium, ortungsten, is preferably used.

The gate insulating film 127 is provided to have a single-layerstructure or a stacked-layer structure using, for example, one or moreof insulating materials such as silicon oxide, silicon oxynitride,silicon nitride oxide, silicon nitride, aluminum oxide, hafnium oxide,gallium oxide, or Ga—Zn-based metal oxide. Note that in order to improvethe characteristics of the interface between the gate insulating film127 and the semiconductor film 111, a region in the gate insulating film127, which is in contact with at least the semiconductor film 111, ispreferably formed using an oxide insulating film.

It is possible to prevent outward diffusion of oxygen from thesemiconductor film 111 to the outside and entry of hydrogen, water, orthe like into the semiconductor film 111 from the outside by providingan insulating film having a barrier property against oxygen, hydrogen,water, and the like for the gate insulating film 127. As for theinsulating film having a barrier property against oxygen, hydrogen,water, and the like, an aluminum oxide film, an aluminum oxynitridefilm, a gallium oxide film, a gallium oxynitride film, an yttrium oxidefilm, an yttrium oxynitride film, a hafnium oxide film, a hafniumoxynitride film, and a silicon nitride film can be given as examples.

The gate insulating film 127 may be formed using a high-k material suchas hafnium silicate (HfSiO_(x)), hafnium silicate containing nitrogen(HfSi_(x)O_(y)N_(z)), hafnium aluminate containing nitrogen(HfAl_(x)O_(y)N_(z)), hafnium oxide, or yttrium oxide, so that gateleakage current of the transistor 103 can be reduced.

Moreover, it is preferable that the gate insulating film 127 have astacked-layer structure including the following: a silicon nitride filmwith a small number of defects as a first silicon nitride film; asilicon nitride film with small amounts of released hydrogen andreleased ammonia as a second silicon nitride film over the first siliconnitride film; and any one of the oxide insulating films given above asexamples of the gate insulating film 127 over the second silicon nitridefilm.

In the second silicon nitride film, in thermal desorption spectrometry,the number of released hydrogen molecules is preferably less than 5×10²¹molecules/cm³, more preferably less than or equal to 3×10²¹molecules/cm³, further preferably less than or equal to 1×10²¹molecules/cm³, and the number of released ammonia molecules ispreferably less than 1×10²² molecules/cm³, more preferably less than orequal to 5×10²¹ molecules/cm³, further preferably less than or equal to1×10²¹ molecules/cm³. The first silicon nitride film and the secondsilicon nitride film are used as part of the gate insulating film 127,so that a gate insulating film with a small number of defects and smallamounts of released hydrogen and released ammonia can be formed as thegate insulating film 127. Accordingly, it is possible to reduce theamount of hydrogen and nitrogen in the gate insulating film 127, whichare transferred to the semiconductor film 111.

In the case where trap levels (also referred to as interface states) arepresent at the interface between the oxide semiconductor film 111 formedusing an oxide semiconductor and the gate insulating film or in the gateinsulating film in the transistor including an oxide semiconductor,change of the threshold voltage, typically change of the thresholdvoltage in the negative direction in the transistor and an increase inthe subthreshold swing (S value) showing a gate voltage needed forchanging the drain current by one digit when the transistor is turned onare caused. Thus, there is a problem in that electrical characteristicsfluctuate among the transistors. For this reason, when, as the gateinsulating film, the silicon nitride film with a small number of defectsis used, and the oxide insulating film is provided in a region of thegate insulating film which is in contact with the semiconductor film111, a negative shift of the threshold voltage and an increase of an Svalue can be suppressed.

The thickness of the gate insulating film 127 is greater than or equalto 5 nm and less than or equal to 400 nm, preferably greater than orequal to 10 nm and less than or equal to 300 nm, more preferably greaterthan or equal to 50 nm and less than or equal to 250 nm.

The semiconductor films 111, 119, and 631 are formed using an oxidesemiconductor. The semiconductor films 111, 119, and 631 can have anamorphous crystal structure, a single crystal structure, or apolycrystalline structure. Further, the semiconductor films 111, 119,and 631 are formed using the same metal element. The thickness of eachof the semiconductor films 111, 119, and 631 is greater than or equal to1 nm and less than or equal to 100 nm, preferably greater than or equalto 1 nm and less than or equal to 50 nm, more preferably greater than orequal to 1 nm and less than or equal to 30 nm, further preferablygreater than or equal to 3 nm and less than or equal to 20 nm.

Further, an oxide semiconductor that can be used for the semiconductorfilms 111, 119, and 631 has an energy gap of greater than or equal to 2eV, preferably greater than or equal to 2.5 eV, more preferably greaterthan or equal to 3 eV. The off-state current of the transistor 103 canbe reduced by using an oxide semiconductor with a wide energy gap inthis manner.

An oxide semiconductor used for the semiconductor films 111, 119, and631 is preferably a metal oxide containing at least indium (In) or zinc(Zn). Alternatively, a metal oxide containing both In and Zn ispreferable. In order to reduce fluctuations in electricalcharacteristics of the transistors including the oxide semiconductor,the oxide semiconductor preferably contains one or more of stabilizersin addition to In and Zn.

As for stabilizers, gallium (Ga), tin (Sn), hafnium (Hf), aluminum (Al),and zirconium (Zr) can be given as examples. As another stabilizer,lanthanoids such as lanthanum (La), cerium (Ce), praseodymium (Pr),neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium(Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm),ytterbium (Yb), and lutetium (Lu) can be given as examples.

As the oxide semiconductor that can be used for the semiconductor films111, 119, and 631, for example, the following can be used: indium oxide,tin oxide, or zinc oxide; an oxide containing two kinds of metals suchas an In—Zn-based oxide, a Sn—Zn-based oxide, an Al—Zn-based oxide, aZn—Mg-based oxide, a Sn—Mg-based oxide, an In—Mg-based oxide, or anIn—Ga-based oxide; an oxide containing three kinds of metals such as anIn—Ga—Zn-based oxide (also referred to as IGZO), an In—Al—Zn-basedoxide, an In—Sn—Zn-based oxide, a Sn—Ga—Zn-based oxide, anAl—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide, an In—Hf—Zn-based oxide,an In—Zr—Zn-based oxide, an In—Ti—Zn-based oxide, In—Sc—Zn-based oxide,an In—Y—Zn-based oxide, an In—La—Zn-based oxide, an In—Ce—Zn-basedoxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, anIn—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide,an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-basedoxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide, anIn—Yb—Zn-based oxide, or an In—Lu—Zn-based oxide; or an oxide containingfour kinds of metals such as an In—Sn—Ga—Zn-based oxide, anIn—Hf—Ga—Zn-based oxide, an In—Al—Ga—Zn-based oxide, anIn—Sn—Al—Zn-based oxide, an In—Sn—Hf—Zn-metal oxide, or anIn—Hf—Al—Zn-based oxide.

Here, an In—Ga—Zn-based oxide means an oxide containing In, Ga, and Znas its main components and there is no particular limitation on theratio of In, Ga, and Zn. The In—Ga—Zn-based oxide may contain a metalelement other than In, Ga, and Zn.

Alternatively, a material represented by InMO₃(ZnO)_(m) (m>0 issatisfied) may be used as the oxide semiconductor. Note that Mrepresents one or more metal elements selected from Ga, Fe, Mn, and Co,or the above element as a stabilizer.

For example, it is possible to use an In—Ga—Zn-based metal oxidecontaining In, Ga, and Zn at an atomic ratio of 1:1:1 (=⅓:⅓:⅓), 2:2:1(=⅖:⅖:⅕), or 3:1:2 (=½:⅙:⅓). Alternatively, an In—Sn—Zn-based metaloxide containing In, Sn, and Zn at an atomic ratio of 1:1:1 (=⅓:⅓:⅓),2:1:3 (=⅓:⅙:½), or 2:1:5 (=¼:⅛:⅝) may be used. Note that a proportion ofeach atom in the atomic ratio of the metal elements contained in themetal oxide varies within a range of ±20% as an error.

Note that without limitation to the materials given above, a materialwith an appropriate atomic ratio depending on semiconductorcharacteristics and electrical characteristics (field-effect mobility,threshold voltage, and the like) may be used. In order to obtainnecessary semiconductor characteristics, it is preferable that thecarrier density, the impurity concentration, the defect density, theatomic ratio of a metal element to oxygen, the interatomic distance, thedensity, and the like be set to be appropriate. For example, highfield-effect mobility can be obtained relatively easily in the case ofusing an In—Sn—Zn-based metal oxide. Also in the case of using anIn—Ga—Zn-based metal oxide, field-effect mobility can be increased byreducing the defect density in a bulk.

The signal line 109 including the source electrode of the transistor103, the conductive film 113 including the drain electrode of thetransistor 103, and the conductive film 125, the source electrode 629,and the drain electrode 633 electrically connecting the semiconductorfilm 119 and the capacitor line 115 in the capacitor 105 can be formedto have a single-layer structure or a layered structure using a materialwhich can be used for the scan line 107, the capacitor line 115, and thegate electrode 627.

The pixel electrode 121 and the conductive film 641 are formed using alight-transmitting conductive material such as an indium tin oxide, anindium oxide containing a tungsten oxide, an indium zinc oxidecontaining a tungsten oxide, an indium oxide containing a titaniumoxide, an indium tin oxide containing a titanium oxide, an indium zincoxide, or an indium tin oxide to which a silicon oxide is added.

Here, connection of the components included in the pixel 101 describedin this embodiment is described with reference to the circuit diagram inFIG. 1C and the cross-sectional view in FIG. 3.

FIG. 1C is an example of a detailed circuit diagram of the pixel 101included in the semiconductor device illustrated in FIG. 1A. Asillustrated in FIG. 1C and FIG. 3, the transistor 103 includes the scanline 107 including the gate electrode, the signal line 109 including thesource electrode, and the conductive film 113 including the drainelectrode.

The semiconductor film 119 which is electrically connected to thecapacitor line 115 through the conductive film 125 serves as oneelectrode of the capacitor 105. Further, the pixel electrode 121 whichis electrically connected to the conductive film 113 including a drainelectrode serves as the other electrode of the capacitor 105. Theinsulating films 129, 131, 132, and 137 provided between thesemiconductor film 119 and the pixel electrode 121 serve as a dielectricfilm.

The liquid crystal element 108 includes the pixel electrode 121, thecounter electrode 154, and the liquid crystal layer provided between thepixel electrode 121 and the counter electrode 154.

Despite having a structure which is the same as that of thesemiconductor film 111, the semiconductor film 119 in the capacitor 105functions as the electrode of the capacitor 105. This is because thepixel electrode 121 can function as a gate electrode, the insulatingfilms 129, 131, 132, and 137 can function as gate insulating films, anda capacitor line 115 can function as a source electrode or a drainelectrode, so that the capacitor 105 can be operated in a manner similarto that of a transistor and the semiconductor film 119 can be made to bein a conductive state. In other words, the capacitor 105 can be a metaloxide semiconductor (MOS) capacitor. Power is supplied to a MOScapacitor when a voltage higher than the threshold voltage (Vth) isapplied to one electrode of the MOS capacitor (the pixel electrode 121of the capacitor 105). Further, the semiconductor film 119 can be madeto be in a conductive state so that the semiconductor film 119 canfunction as one electrode of the capacitor by controlling a potential tobe supplied to the capacitor line 115. In this case, the potential to besupplied to the capacitor line 115 is set as follows. The potential ofthe pixel electrode 121 is changed in the positive direction and thenegative direction relative to the center potential of a video signal inorder to operate the liquid crystal element 108 (see FIG. 1C). Thepotential of the capacitor line 115 needs to be constantly lower thanthe potential to be supplied to the pixel electrode 121 by the thresholdvoltage (Vth) of the capacitor 105 (MOS capacitor) or more in order thatthe capacitor 105 (MOS capacitor) be constantly in a conductive state.In other words, since the semiconductor film 119 has the same structureas the semiconductor film 111, the potential of the capacitor line 115should be lower than the potential to be supplied to the pixel electrode121 by the threshold voltage of the transistor 103 or more. In such amanner, the semiconductor film 119 can be made to be constantly in aconductive state; thus, the capacitor 105 (MOS capacitor) can be made tobe in a conductive state.

When an oxide insulating film that oxygen permeates and which has a lowinterface state density at the interfaces with the semiconductor films111 and 631 is used as the insulating film 129 over the semiconductorfilms 111 and 631 and an oxide insulating film which includes an oxygenexcess region or an oxide insulating film in which the oxygen content ishigher than that in the stoichiometric composition is used as theinsulating film 131, oxygen can be easily supplied to the oxidesemiconductor film as the semiconductor films 111 and 631 formed usingan oxide semiconductor, the release of oxygen from the semiconductorfilms 111 and 631 can be prevented, and the oxygen contained in theinsulating film 131 can enter the semiconductor films 111 and 631 toreduce oxygen vacancies in the oxide semiconductor included in thesemiconductor films 111 and 631. Thus, the transistor 103 can beprevented from being normally on and a potential to be supplied to thecapacitor line 115 can be controlled so that the capacitor 105 (MOScapacitor) can be constantly in a conductive state; thus, thesemiconductor device can have favorable electrical characteristics andhigh reliability.

The use of a nitride insulating film as the insulating film 132 over theinsulating film 131 can suppress entry of impurities such as hydrogenand water into the semiconductor films 111 and 119 from the outside.Moreover, the use of a nitride insulating film with a low hydrogencontent as the insulating film 132 can minimize variations in electricalcharacteristics of the transistor and the capacitor 105 (MOS capacitor).

Further, the capacitor 105 can be formed large (in a large area) in thepixel 101. For this reason, the semiconductor device can have chargecapacity increased while improving the aperture ratio. Accordingly, thesemiconductor device can have excellent display quality.

<Method for Manufacturing Semiconductor Device>

Next, a formation method of the element portion over the substrate 102in the semiconductor device described above is described with referenceto FIGS. 4A and 4B and FIGS. 5A and 5B.

First, the scan line 107, the capacitor line 115, and the gate electrode627 are formed over the substrate 102. An insulating film 126 which isto be processed into the gate insulating film 127 later is formed so asto cover the scan line 107, the capacitor line 115, and the gateelectrode 627. The semiconductor film 111 is formed over a portion ofthe insulating film 126 which overlaps with the scan line 107. Thesemiconductor film 119 is formed so as to overlap with a region wherethe pixel electrode 121 is to be formed later. The semiconductor film631 is formed in a region overlapping with the gate electrode 627 (seeFIG. 4A).

The scan line 107, the capacitor line 115, and the gate electrode 627can be formed in such a manner that a conductive film is formed usingany of the materials given above, a mask is formed over the conductivefilm, and processing is performed using the mask. The conductive filmcan be formed by any of a variety of deposition methods such as anevaporation method, a CVD method, a sputtering method, and a spincoating method. Note that there is no particular limitation on thethickness of the conductive film, and the thickness of the conductivefilm can be determined in consideration of time needed for theformation, desired resistivity, or the like. As the mask, a resist maskformed through a photolithography process can be used. The conductivefilm can be processed by one or both of dry etching and wet etching.

The insulating film 126 can be formed using a material which can be usedfor the gate insulating film 127, by any of a variety of depositionmethods such as a CVD method and a sputtering method.

In the case where a gallium oxide is used for the gate insulating film127, the insulating film 126 can be formed by a metal organic chemicalvapor deposition (MOCVD) method.

The semiconductor films 111, 119, and 631 can be formed in such a mannerthat an oxide semiconductor film using any of the oxide semiconductorslisted above is formed, a mask is formed over the oxide semiconductorfilm, and processing is performed using the mask. Thus, thesemiconductor films 111, 119, and 631 are formed using the same metalelement. The oxide semiconductor film can be formed by a sputteringmethod, a coating method, a pulsed laser deposition method, a laserablation method, or the like. Alternatively, when a printing method isemployed, the semiconductor films 111 and 119 which are separate fromeach other can be formed directly on the insulating film 126. In thecase where the oxide semiconductor film is formed by a sputteringmethod, an RF power supply device, an AC power supply device, a DC powersupply device, or the like can be used as appropriate as a power supplydevice for generating plasma. As a sputtering gas, a rare gas (typicallyargon), an oxygen gas, or a mixed gas of a rare gas and oxygen is usedas appropriate. In the case of using the mixed gas of a rare gas andoxygen, the proportion of oxygen is preferably higher than that of arare gas. Further, a target may be appropriately selected in accordancewith the composition of the oxide semiconductor film to be formed. Asthe mask, a resist mask formed through a photolithography process can beused. The conductive film can be processed by one or both of dry etchingand wet etching. The etching conditions (e.g., an etching gas or anetching solution, etching time, and temperature) are set as appropriatedepending on the material so that the oxide semiconductor film can beetched to have a desired shape.

Heat treatment is preferably performed after the semiconductor films111, 119, and 631 are formed so that the semiconductor films 111, 119,and 631 are subjected to dehydrogenation or dehydration. The temperatureof the heat treatment is typically higher than or equal to 150° C. andlower than the strain point of the substrate, preferably higher than orequal to 200° C. and lower than or equal to 450° C., further preferablyhigher than or equal to 300° C. and lower than or equal to 450° C. Notethat the heat treatment may be performed on the oxide semiconductor filmwhich has not been processed into the semiconductor films 111 and 119.

A heat treatment apparatus used in the heat treatment is not limited toan electric furnace; as the heat treatment apparatus, an apparatus whichheats an object using thermal conduction or thermal radiation given by amedium such as a heated gas may be used. For example, an RTA (rapidthermal anneal) apparatus such as a GRTA (gas rapid thermal anneal)apparatus or an LRTA (lamp rapid thermal anneal) apparatus can be used.An LRTA apparatus is an apparatus for heating an object to be processedby radiation of light (an electromagnetic wave) emitted from a lamp suchas a halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arclamp, a high pressure sodium lamp, or a high pressure mercury lamp. AGRTA apparatus is an apparatus for heat treatment using ahigh-temperature gas.

The heat treatment may be performed in an atmosphere of nitrogen,oxygen, ultra-dry air (air in which the water content is 20 ppm or less,preferably 1 ppm or less, further preferably 10 ppb or less), or a raregas (argon, helium, or the like). The atmosphere of nitrogen, oxygen,ultra-dry air, or a rare gas preferably does not contain hydrogen,water, and the like. Alternatively, the heat treatment may be performedunder an atmosphere of an inert gas first, and then under an oxygenatmosphere. The treatment time is 3 minutes to 24 hours.

In the case where a base insulating film is provided between thesubstrate 102, and the scan line 107, the capacitor line 115, and thegate insulating film 127, the base insulating film can be formed usingany of the following: silicon oxide, silicon oxynitride, siliconnitride, silicon nitride oxide, gallium oxide, hafnium oxide, yttriumoxide, aluminum oxide, aluminum oxynitride, and the like. The use ofsilicon nitride, gallium oxide, hafnium oxide, yttrium oxide, aluminumoxide, or the like for the base insulating film leads to suppression ofdiffusion of impurities typified by an alkali metal, water, and hydrogeninto the semiconductor films 111, 119, and 631 from the substrate 102.The base insulating film can be formed by a sputtering method or a CVDmethod.

After the opening 123 reaching the capacitor line 115 is formed in theinsulating film 126 to form the gate insulating film 127, the signalline 109 including the source electrode of the transistor 103, theconductive film 113 including the drain electrode of the transistor 103,and the conductive film 125 which electrically connects thesemiconductor film 119 and the capacitor line 115 are formed. Further,the source electrode 629 and the drain electrode 633 are formed (seeFIG. 4B).

The opening 123 can be formed in such a manner that a mask is formed soas to expose part of a portion of the insulating film 126 which overlapswith the capacitor line 115 and processing is performed using the mask.The formation of the mask and the processing can be performed in mannerssimilar to those of the scan line 107, the capacitor line 115, and thegate electrode 627.

The signal line 109, the conductive films 113 and 125, the sourceelectrode 629, and the drain electrode 633 can be formed as follows: aconductive film is formed using a material which can be used for thesignal line 109, the conductive films 113 and 125, and the gateelectrode 627, a mask is formed over the conductive film, and processingis performed using the mask. The formation of the mask and theprocessing can be performed in manners similar to those of the scan line107, the capacitor line 115, and the gate electrode 627.

Next, an insulating film 128 is formed over the semiconductor films 111,119, and 631, the signal line 109, the conductive films 113 and 125, thesource electrode 629, the drain electrode 633, and the gate insulatingfilm 127. An insulating film 130 is formed over the insulating film 128.An insulating film 133 is formed over the insulating film 130. Aninsulating film 136 is formed over the insulating film 133 (see FIG.5A). Note that it is preferable that the insulating films 128, 130, and133 be formed successively. In such a case, entry of impurities intoeach interface can be suppressed.

The insulating film 128 can be formed using a material which can be usedfor the insulating film 129, by any of a variety of deposition methodssuch as a CVD method and a sputtering method. The insulating film 130can be formed using a material which can be used for the insulating film131, by any of a variety of deposition methods such as a CVD method anda sputtering method. The insulating film 133 can be formed using amaterial which can be used for the insulating film 132, by any of avariety of deposition methods such as a CVD method and a sputteringmethod. The insulating film 136 can be formed using a material which canbe used for the insulating film 137 by a CVD method.

In the case where an oxide insulating film which has a low interfacestate density between the semiconductor film 111 and the oxideinsulating film is used as the insulating film 129, the insulating film128 can be formed under the following formation conditions. Here, as theoxide insulating film, a silicon oxide film or a silicon oxynitride filmis formed. As for the formation conditions, the substrate placed in atreatment chamber of a plasma CVD apparatus, which is vacuum-evacuated,is held at a temperature higher than or equal to 180° C. and lower thanor equal to 400° C., preferably higher than or equal to 200° C. andlower than or equal to 370° C., a deposition gas containing silicon andan oxidizing gas are introduced as a source gas into the treatmentchamber, the pressure in the treatment chamber is greater than or equalto 20 Pa and less than or equal to 250 Pa, preferably greater than orequal to 40 Pa and less than or equal to 200 Pa, and high-frequencypower is supplied to an electrode provided in the treatment chamber.

Typical examples of the deposition gas containing silicon includesilane, disilane, trisilane, silane fluoride, and the like. Examples ofthe oxidizing gas include oxygen, ozone, dinitrogen monoxide, andnitrogen dioxide.

By setting the ratio of the amount of the oxidizing gas to the amount ofthe deposition gas containing silicon to 100 or higher, the hydrogencontent in the insulating film 128 (the insulating film 129) can bereduced and dangling bonds in the insulating film 128 (the insulatingfilm 129) can be reduced. Oxygen released from the insulating film 130(the insulating film 131) is captured by the dangling bonds in theinsulating film 128 (the insulating film 129) in some cases; thus, inthe case where the dangling bonds in the insulating film 128 (theinsulating film 129) are reduced, oxygen in the insulating film 130 (theinsulating film 131) can enter the semiconductor films 111 and 119efficiently to reduce the oxygen vacancies in the oxide semiconductorsused to form the semiconductor films 111 and 119. As a result, theamount of hydrogen entering the semiconductor films 111 and 119 can bereduced and oxygen vacancies in the semiconductor films 111 and 119 canbe reduced.

In the case where the above oxide insulating film which includes anoxygen excess region or the above oxide insulating film in which theoxygen content is higher than that in the stoichiometric composition isused as the insulating film 131, the insulating film 130 can be formedunder the following formation conditions. Here, as the oxide insulatingfilm, a silicon oxide film or a silicon oxynitride film is formed. Asfor the formation conditions, the substrate placed in a treatmentchamber of a plasma CVD apparatus, which is vacuum-evacuated, is held ata temperature higher than or equal to 180° C. and lower than or equal to260° C., preferably higher than or equal to 180° C. and lower than orequal to 230° C., a source gas is introduced into the treatment chamber,the pressure in the treatment chamber is greater than or equal to 100 Paand less than or equal to 250 Pa, preferably greater than or equal to100 Pa and less than or equal to 200 Pa, and high-frequency power thatis higher than or equal to 0.17 W/cm² and lower than or equal to 0.5W/cm², preferably, higher than or equal to 0.25 W/cm² and lower than orequal to 0.35 W/cm² is supplied is supplied to an electrode provided inthe treatment chamber.

As the source gas of the insulating film 130, a source gas which can beused for the insulating film 128 can be used.

As for the formation conditions of the insulating film 130, thehigh-frequency power having the above power density is supplied to thetreatment chamber having the above pressure, whereby the decompositionefficiency of the source gas in plasma is increased, oxygen radicals areincreased, and oxidation of the source gas proceeds; therefore, theoxygen content in the insulating film 130 is higher than that in thestoichiometric composition. In addition, in the film formed at asubstrate temperature within the above temperature range, the bondbetween silicon and oxygen is weak, and accordingly, part of oxygen inthe film can be released by heat treatment in the later step. Thus, itis possible to form an oxide insulating film which contains oxygen at ahigher proportion than the stoichiometric composition and from whichpart of oxygen is released by heating. The insulating film 128 isprovided over the semiconductor film 111. Accordingly, in the processfor forming the insulating film 130, the insulating film 128 serves as aprotective film of the semiconductor film 111. Thus, even when theinsulating film 130 is formed using the high-frequency power having ahigh power density, damage to the semiconductor films 111 and 631 is notsignificant.

By increasing the thickness of the insulating film 130, a larger amountof oxygen is released by heating; thus, the insulating film 130 ispreferably formed thicker than the insulating film 128. Since theinsulating film 128 is provided, favorable coverage can be achieved evenwhen the insulating film 130 is formed thick.

The insulating film 132 can be formed by a sputtering method, a CVDmethod, or the like. In the case where a nitride insulating film with alow hydrogen content is used as the insulating film 132, the insulatingfilm 133 can be formed under the following formation conditions. Here,as the nitride insulating film, a silicon nitride film is formed. Thesubstrate placed in a treatment chamber of the plasma CVD apparatus,which is vacuum-evacuated, is held at a temperature higher than or equalto 80° C. and lower than or equal to 400° C., preferably higher than orequal to 200° C. and lower than or equal to 370° C., the pressure isgreater than or equal to 100 Pa and less than or equal to 250 Pa,preferably greater than or equal to 100 Pa and less than or equal to 200Pa with introduction of a source gas into the treatment chamber, andhigh-frequency power is supplied to an electrode provided in thetreatment chamber.

As the source gas of the insulating film 132, a deposition gascontaining silicon, a nitrogen gas, and an ammonia gas are preferablyused. Typical examples of the deposition gas containing silicon includesilane, disilane, trisilane, silane fluoride, and the like. Further, theflow rate of nitrogen is preferably 5 times to 50 times that of ammonia,further preferably 10 times to 50 times that of ammonia. The use ofammonia as the source gas facilitates decomposition of nitrogen and thedeposition gas containing silicon. This is because ammonia isdissociated by plasma energy or heat energy, and energy generated by thedissociation contributes to decomposition of a bond of the depositiongas molecules containing silicon and a bond of nitrogen molecules. Underthe above conditions, a silicon nitride film which has a low hydrogencontent and can suppress entry of impurities such as hydrogen and waterfrom the outside can be formed.

The insulating film 136 is formed using an organosilane gas and oxygenby a CVD method at a substrate temperature of higher than or equal to200° C. and lower than or equal to 550° C., preferably higher than orequal to 300° C. and lower than or equal to 450° C.

It is preferable that heat treatment be performed at least afterformation of the insulating film 130 so that excess oxygen contained inthe insulating film 128 or the insulating film 130 enters thesemiconductor films 111 and 631 to reduce oxygen vacancies in the oxidesemiconductor included in the semiconductor films 111 and 631. The heattreatment can be appropriately performed according to the details ofheat treatment for dehydration or dehydrogenation of the semiconductorfilms 111 and 119.

Next, the opening 117 (see FIG. 2) reaching the conductive film 113 isformed in regions of the insulating films 128, 130, 133, and 136 whichoverlap with the conductive film 113, and the insulating films 129, 131,132, and 137 are formed. The opening 117 (see FIG. 2) can be formed in amanner similar to that of the opening 123.

Finally, the pixel electrode 121 and the conductive film 641 are formed,so that the element portion over the substrate 102 can be formed (seeFIG. 5B). The pixel electrode 121 is formed in such a manner that aconductive film is formed using any of the materials given above incontact with the conductive film 113 through the opening 117, a mask isformed over the conductive film, and processing is performed using themask. The formation of the mask and the processing can be performed inmanners similar to those of the scan line 107 and the capacitor line115.

Modification Example 1

In the semiconductor device that is an embodiment of the presentinvention, connection of the capacitor line 115 and the semiconductorfilm 119 serving as one electrode of the capacitor can be changed asappropriate. For example, to improve the aperture ratio, a structurewhere the semiconductor film is in direct contact with the capacitorline without the conductive film 125 interposed therebetween can beemployed. The conductive film 125 functioning as a light-blocking filmis not formed; thus, the aperture ratio of the pixel can be furtherincreased.

Modification Example 2

In the semiconductor device that is an embodiment of the presentinvention, the top shape of the conductive film 125 which electricallyconnects the capacitor line 115 and the semiconductor film 119 servingas one electrode of the capacitor 105 can be changed as appropriate. Forexample, to reduce contact resistance between the semiconductor film 119and the conductive film 125, the conductive film 125 can be provided incontact with the semiconductor film 119 along the outer peripherythereof. The conductive film is formed in the same formation process asthe signal line 109 including the source electrode of the transistor 103and the conductive film 113 including the drain electrode of thetransistor 103 and thus may have a light-blocking property; for thisreason, the conductive film is preferably formed into a loop shape.

Modification Example 3

In the semiconductor device that is an embodiment of the presentinvention, the structures of the semiconductor film included in thecapacitor and the capacitor line can be changed as appropriate. Aspecific example of the structure is described with reference to FIG. 6.FIG. 6 is a top view of the pixel 172. In the pixel 172 illustrated inFIG. 6, a side parallel to the scan line 107 is longer than a sideparallel to the signal line 109, and the capacitor line 176 may beprovided so as to extend in the direction parallel to the signal line109. The signal line 109 and the capacitor line 176 are electricallyconnected to the signal line driver circuit 106 (see FIG. 1A).

A capacitor 174 is connected to the capacitor line 176 provided so as toextend in the direction parallel to the signal line 109. The capacitor174 includes a semiconductor film 178 including an oxide semiconductorand formed in the same process as the semiconductor film 111, the pixelelectrode 121, and an insulating film (not illustrated in FIG. 6) whichis formed over the transistor 103, as a dielectric film. Thesemiconductor film 111, the pixel electrode 121, and the dielectric filmtransmit light; accordingly, the capacitor 174 transmits light.

Note that in the capacitor 174, the dielectric film provided between thepair of electrodes includes the insulating films 129, 131, 132, and 137,similarly to the capacitor 105 illustrated in FIG. 3.

The capacitor line 176 can be formed in the same process as the signalline 109 and the conductive film 113. When the capacitor line 176 isprovided in contact with the semiconductor film 178, the area where thesemiconductor film 178 and the capacitor line 176 are in contact witheach other can be increased. A side of the pixel 172 parallel to thesignal line 109 is shorter than a side of the pixel 172 parallel to thescan line 107; thus, the area where the pixel electrode 121 overlapswith the capacitor line 176 can be decreased, resulting in a higheraperture ratio.

Modification example 4

In the semiconductor device that is an embodiment of the presentinvention, one electrode of the capacitor and the capacitor line can beformed using a semiconductor film. A specific example is described withreference to FIG. 7. Here, only a semiconductor film 198 different fromthe semiconductor film 119 and the capacitor line 115 described withreference to FIG. 2 is described. FIG. 7 shows a top view of a pixel 196where the semiconductor film 198 serving as one electrode of a capacitor197 and the capacitor line is provided in the pixel 196. Thesemiconductor film 198 has a region which extends in the directionparallel to the signal line 109 and the region functions as thecapacitor line. In the semiconductor film 198, a region which overlapswith the pixel electrode 121 functions as one electrode of the capacitor197. The semiconductor film 198 can be formed concurrently with thesemiconductor film 111 of the transistor 103 provided in the pixel 196.

In the case where a continuous semiconductor film is provided as thesemiconductor film 198 for the pixels 196 in one row, the semiconductorfilm 198 overlaps with the scan lines 107. For this reason, thesemiconductor film 198 does not function as the capacitor line and theone electrode of the capacitor 197 due to an effect of a change in thepotential of the scan line 107 in some cases. Thus, it is preferablethat, as illustrated in FIG. 7, the semiconductor film 198 of one pixel196 be positioned separately from that of another pixel 196 andsemiconductor films 198 be electrically connected to each other by aconductive film 199 which can be formed at the same time as the signalline 109 and the conductive film 113. With the above structure, aportion of the semiconductor film 198 which is not connected to theconductive film 199 overlaps with the pixel electrode 121, whereby theresistance of the semiconductor film 198 in the region can be low andthus the semiconductor film 198 functions as the capacitor line and oneelectrode of the capacitor 197.

Although not illustrated, one semiconductor film can be provided as thesemiconductor film 198 for the pixels 196 so as to overlap with the scanlines 107 in the case where a portion of the semiconductor film 198which overlaps with the scan line 107 is not influenced by a change inthe potential of the scan line 107. In other words, a continuous oxidesemiconductor film can be provided as the semiconductor film 198 for thepixels 196 in one row.

In FIG. 7, a portion of the semiconductor film 198 which functions asthe capacitor line extends in the direction parallel to the signal line109; however, the region which functions as the capacitor line mayextend in the direction parallel to the scan line 107. In the case wherethe portion of the semiconductor film 198 which functions as thecapacitor line extends in the direction parallel to the scan line 107,it is necessary that the semiconductor film 111 and the semiconductorfilm 198 be electrically insulated from the signal line 109 and theconductive film 113 by providing an insulating film between thesemiconductor film 111 and the semiconductor film 198, and the signalline 109 and the conductive film 113, in the transistor 103 and thecapacitor 197.

According to the above description, when a light-transmitting oxidesemiconductor film is provided for one electrode of a capacitor providedin a pixel and a capacitor line as in the pixel 196, the pixel can havea higher aperture ratio.

Modification Example 5

In the semiconductor device that is an embodiment of the presentinvention, the structure of the capacitor line can be changed asappropriate. This structure is described with reference to FIG. 8. InFIG. 8, unlike the capacitor line 115 described with reference to FIG.2, a capacitor line is located between adjacent two pixels.

FIG. 8 illustrates a structure where the capacitor line is providedbetween the pixels adjacent to each other in the direction in which asignal line 409 extends. In stead of the structure of FIG. 8, astructure where a capacitor line is provided between pixels adjacent toeach other in the direction in which a scan line 437 extends may be usedas appropriate.

FIG. 8 is a top view of a pixel 401_1 and a pixel 401_2 adjacent to eachother in the direction in which the signal line 409 extends.

A scan line 407_1 and a scan line 407_2 are provided so as to extend inparallel to each other in the direction substantially perpendicular tothe signal line 409. A capacitor line 415 is provided between the scanlines 407_1 and 407_2 so as to be in parallel to the scan lines 407_1and 407_2. The capacitor line 415 is electrically connected to acapacitor 405_1 provided in the pixel 401_1 and a capacitor 4052provided in the pixel 401_2. The top shapes and the positions ofcomponents of the pixel 401_1 and those of the pixel 401_2 are symmetricwith respect to the capacitor line 415.

The pixel 401_1 is provided with a transistor 403_1 and a pixelelectrode 421_1 and the capacitor 405_1 which are electrically connectedto the transistor 403_1.

The transistor 403_1 is provided in a region where the scan line 407_1and the signal line 409 intersect with each other. The transistor 403_1includes at least an oxide semiconductor film 411_1 including a channelformation region, a gate electrode, a gate insulating film (notillustrated in FIG. 8), a source electrode, and a drain electrode. Aregion of the scan line 407_1 which overlaps with the semiconductor film411_1 serves as the gate electrode of the transistor 403_1. A region ofthe signal line 409 which overlaps with the semiconductor film 411_1serves as the source electrode of the transistor 403_1. A region of aconductive film 413_1 which overlaps with the semiconductor film 411_1serves as the drain electrode of the transistor 403_1. The conductivefilm 413_1 and the pixel electrode 421_1 are electrically connected toeach other through an opening 417_1.

The capacitor 405_1 is electrically connected to the capacitor line 415through a conductive film 425 provided in and over an opening 423. Thecapacitor 405_1 includes a semiconductor film 419_1 formed with an oxidesemiconductor, the pixel electrode 421_1, and, as a dielectric film, aninsulating film (not illustrated in FIG. 8) which is included in thetransistor 403_1. The semiconductor film 419_1, the pixel electrode421_1, and the dielectric film transmit light; thus, the capacitor 405_1transmits light.

The pixel 401_2 is provided with a transistor 403_2 and a pixelelectrode 421_2 and the capacitor 405_2 which are electrically connectedto and the transistor 403_2.

The transistor 403_2 is provided in a region where the scan line 407_2and the signal line 409 intersect with each other. The transistor 403_2includes at least a semiconductor film 411_2 including a channelformation region, a gate electrode, a gate insulating film (notillustrated in FIG. 8), a source electrode, and a drain electrode. Aregion of the scan line 407_2 which overlaps with the semiconductor film411_2 serves as the gate electrode of the transistor 403_2. A region ofthe signal line 409 which overlaps with the semiconductor film 411_2serves as the source electrode of the transistor 403_2. A region of theconductive film 413_2 which overlaps with the semiconductor film 411_2serves as the drain electrode of the transistor 403_2. The conductivefilm 413_2 and the pixel electrode 421_2 are connected to each otherthrough an opening 417_2.

The capacitor 405_2 is electrically connected to the capacitor line 415through the conductive film 425 provided in and over the opening 423 ina manner similar to that of the capacitor 405_1. The capacitor 405_2includes a semiconductor film 419_2 formed with an oxide semiconductor,the pixel electrode 421_2, and, as a dielectric film, an insulating film(not illustrated in FIG. 8) which is formed in the transistor 403_2. Thesemiconductor film 419_2, the pixel electrode 421_2, and the dielectricfilm transmit light; thus, the capacitor 405_2 transmits light.

Cross-sectional structures of the transistors 403_1 and 403_2 and thecapacitors 405_1 and 405_2 are similar to those of the transistor 103and the capacitor 105 illustrated in FIG. 3 and thus descriptionsthereof are omitted here.

In a structure seen from above, a capacitor line is provided betweenadjacent two pixels so that capacitors included in the pixels and thecapacitor line are connected, whereby the number of capacitor lines canbe reduced. As a result, the aperture ratio of the pixel can be high ascompared with the case of a structure where each pixel is provided witha capacitor line.

Modification Example 6

In the semiconductor device that is an embodiment of the presentinvention, the shape of a transistor provided in a pixel is not limitedto the shape of the transistor illustrated in FIG. 2 and FIG. 3 and canbe changed as appropriate. For example, in the transistor, a sourceelectrode included in the signal line 109 has a U shape (or a C shape, asquare-bracket-like shape, or a horseshoe shape) which surrounds theconductive film including a drain electrode. With such a shape, anenough channel width can be ensured even when the area of the transistoris small, and accordingly, the amount of drain current flowing at thetime of conduction of the transistor (also referred to as an on-statecurrent) can be increased.

Modification Example 7

Although in the pixels 101, 172, and 196 described above, a transistorwhere the semiconductor film is provided between the gate insulatingfilm, and the signal line 109 including the source electrode and theconductive film 113 including the drain electrode is used, instead ofthe transistor, a transistor where a semiconductor film is providedbetween the insulating film 129, and the signal line including thesource electrode and the conductive film including the drain electrodecan be used.

Modification Example 8

Although in the pixels 101, 172, and 196 described above, achannel-etched transistor is used as the transistor, instead of thetransistor, a channel protective transistor can be used. Further, when achannel protective film is provided, a surface of the semiconductor film111 is not exposed to an etchant or an etching gas used in a formationprocess of the signal line and the conductive film, so that impuritiesbetween the semiconductor film 111 and the channel protective film canbe reduced. Accordingly, a leakage current flowing between the sourceelectrode and the drain electrode of the transistor can be reduced.

Modification Example 9

Although in the pixels 101, 172, 196, 401_1, and 401_2 described above,a transistor having one gate electrode is used, instead of thetransistor, a transistor having two gate electrodes facing each otherwith the semiconductor film 111 interposed therebetween can be used.

The transistor includes a conductive film over the insulating film 137of the transistor 103 described in this embodiment. The conductive filmoverlaps with at least a channel formation region of the semiconductorfilm 111. It is preferable that the conductive film be provided in aposition overlapping with the channel formation region of thesemiconductor film 111 so that the potential of the conductive film isequal to the minimum potential of a video signal input to the signalline 109. In that case, a current flowing between the source electrodeand the drain electrode in a surface of the semiconductor film 111facing the conductive film 187 can be controlled, and variations in theelectrical characteristics of the transistors can be reduced. Further,when the conductive film is provided, an influence of a change inambient electric field on the semiconductor film 111 can be reduced,leading to an improvement in reliability of the transistor.

The conductive film can be formed using a material and a method similarto those of the scan line 107, the signal line 109, the pixel electrode121, or the like.

As described above, the use of the semiconductor film formed in the sameformation step as the semiconductor film included in the transistor, forone electrode of the capacitor, allows fabrication of a semiconductordevice including the capacitor whose charge capacity is increased whileimproving the aperture ratio. Accordingly, the semiconductor device canhave excellent display quality.

Further, oxygen vacancies and impurities such as hydrogen in the oxidesemiconductor film, which is formed using an oxide semiconductorincluded in the transistor, are reduced, so that the semiconductordevice that is an embodiment of the present invention has favorableelectrical characteristics.

Note that the structures and the like described in this embodiment canbe combined as appropriate with any of the structures and the likedescribed in the other embodiments and example.

Embodiment 2

In this embodiment, a semiconductor device of an embodiment of thepresent invention, which has a structure different from those describedin the above embodiment, is described with reference to drawings. Notethat in this embodiment, a liquid crystal display device is described asan example of the semiconductor device of an embodiment of the presentinvention. In the semiconductor device described in this embodiment, thestructure of a capacitor is different from that of the capacitor in theabove embodiment. The above embodiment can be referred to for componentsin the semiconductor device in this embodiment, which are similar tothose of the semiconductor device in the above embodiment.

<Structure of Semiconductor Device>

FIG. 9 is a top view of the pixel 101 in this embodiment. In a pixel 201illustrated in FIG. 9, an insulating film 229 (not illustrated) and aninsulating film 231 (not illustrated) are not provided in a regionindicated by an alternate long and short dashed line. Edges of theinsulating film 229 (not illustrated) and the insulating film 231 (notillustrated) are over the semiconductor film 119. Thus, a capacitor 205in the pixel 201 in FIG. 9 includes the semiconductor film 119 which isone electrode, a pixel electrode 221 serving as the other electrode, andinsulating films 232 and 237 (not illustrated) which serve as adielectric film.

Next, FIG. 10 illustrates a cross-sectional view of a transistorprovided in the scan line driver circuit 104 (see FIG. 1A) andcross-sectional views taken along dashed-dotted line A1-A2,dashed-dotted line B1-B2, and dashed-dotted line C1-C2 in FIG. 9. Here,a top view of the scan line driver circuit 104 is omitted, and across-sectional view of the scan line driver circuit 104 taken alongdashed-dotted line D1-D2 is illustrated. The cross-sectional view of thetransistor provided in the scan line driver circuit 104 is illustratedhere, and the transistor can be provided in the signal line drivercircuit 106.

The cross-sectional structure of the pixel 201 of this embodiment is asfollows. Over the substrate 102, the scan line 107 including the gateelectrode of the transistor 103 and the capacitor line 115 which is onthe same surface as the scan line 107 are provided. The gate insulatingfilm 127 is provided over the scan line 107 and the capacitor line 115.The semiconductor film 111 is provided over a region of the gateinsulating film 127, which overlaps with the scan line 107, and asemiconductor film 219 is provided over the gate insulating film 127.The signal line 109 including the source electrode of the transistor 103and the conductive film 113 serving as the drain electrode of thetransistor 103 are provided over the semiconductor film 111 and the gateinsulating film 127. The opening 123 reaching the capacitor line 115 isprovided in the gate insulating film 127, and the conductive film 125 isprovided in and over the opening 123 and over the gate insulating film127 and the semiconductor film 219. The insulating films 229, 231, 232,and 237 which each serve as a protective insulating film of thetransistor 103 are provided over the gate insulating film 127, thesignal line 109, the semiconductor film 111, the conductive films 113and 125, and the semiconductor film 219. Further, the insulating film232 is provided over the semiconductor film 219 at least in a region tobe the capacitor 205. The opening 117 (see FIG. 9) reaching theconductive film 113 is provided in the insulating films 229, 231, 232and 237, and the pixel electrode 221 is provided in and over the opening117 (see FIG. 9) and over the insulating film 232. Note that a baseinsulating film may be provided between the substrate 102, and the scanline 107, the capacitor line 115, and the gate insulating film 127.

The insulating film 229 is similar to the insulating film 129 describedin Embodiment 1. The insulating film 231 is similar to the insulatingfilm 131 described in Embodiment 1. The insulating film 232 is similarto the insulating film 132 described in Embodiment 1. The insulatingfilm 237 is similar to the insulating film 137 described inEmbodiment 1. The pixel electrode 221 is similar to the pixel electrode121 described in Embodiment 1.

When the insulating films 232 and 237 serve as a dielectric film betweenthe semiconductor film 219 which is one electrode and the pixelelectrode 221 serving as the other electrode as in the capacitor 205 andthe insulating film 237 in this embodiment, the thickness of thedielectric film can be smaller than that of the dielectric film of thecapacitor 105 in Embodiment 1. Thus, the capacitor 205 in thisembodiment can have larger charge capacity than the capacitor 105 inEmbodiment 1.

Further, the insulating films 232 and 237 together serve as a dielectricfilm of the capacitor 205. The insulating film 232 is formed using anitride insulating film which tends to have a higher relative dielectricconstant and a greater internal stress than an oxide insulating filmsuch as a silicon oxide film. Thus, in the case where the insulatingfilm 232 is used alone as the dielectric film of the capacitor 205without the insulating film 237, if the thickness of the insulating film232 is small, the capacitance value of the capacitor 205 becomes toolarge, which makes it difficult to increase the speed of writing animage signal to a pixel with low power consumption. On the other hand,if the thickness of the insulating film 232 is large, internal stressbecomes too large, which leads to a degradation of electricalcharacteristics such as a change in threshold voltage of the transistor.When internal stress of the insulating film 232 becomes too large, theinsulating film 232 is likely to be separated from the substrate 102, sothat the yield is lowered. The insulating film 237 formed using an oxideinsulator such as silicon oxide, which has a lower relative dielectricconstant than the insulating film 232, is used together with theinsulating film 232 as the dielectric film of the capacitor included inthe pixel, so that the dielectric constant of the dielectric film can beadjusted to a desirable value without increasing the thickness of theinsulating film 232.

The insulating film 232 is a nitride insulating film as well as theinsulating film 132 in Embodiment 1, and thus the semiconductor film 219has a high conductivity and becomes n-type. Further, the semiconductorfilm 219 is a light-transmitting conductive film including a metal oxidehaving characteristics of a conductor. The semiconductor film 219 has ahigher conductivity than the semiconductor film 111.

The semiconductor film 219 has a region with higher conductivity thanthat of the semiconductor film 111. With this structure, a portion ofthe semiconductor film 219 which is in contact with the insulating film232 is n-type and has higher conductivity than a portion of thesemiconductor film 111 which is in contact with the insulating film 229.

Note that it is preferable that the semiconductor film 219 have a higherhydrogen concentration than the semiconductor film 111. In thesemiconductor film 219, the hydrogen concentration measured by secondaryion mass spectrometry (SIMS) is greater than or equal to 8×10¹⁹atoms/cm³, preferably greater than or equal to 1×10²⁰ atoms/cm³, morepreferably greater than or equal to 5×10²⁰ atoms/cm³. In thesemiconductor film 111, the hydrogen concentration measured by SIMS isless than 5×10¹⁹ atoms/cm³, preferably less than 5×10¹⁸ atoms/cm³, morepreferably less than or equal to 1×10¹⁸ atoms/cm³, still more preferablyless than or equal to 5×10¹⁷ atoms/cm³, further preferably less than orequal to 1×10¹⁶ atoms/cm³.

The resistivity of the semiconductor film 219 is lower than that of thesemiconductor film 111. The resistivity of the semiconductor film 219 ispreferably greater than or equal to 1×10⁻⁸ times and less than or equalto 1×10⁻¹ times the resistivity of the semiconductor film 111. Theresistivity of the semiconductor film 219 is typically greater than orequal to 1×10⁻³ Ωcm and less than 1×10⁴ Ωcm, preferably greater than orequal to 1×10⁻³ Ωcm and less than 1×10⁻¹ Ωcm.

In an operation method of the capacitor 205 in the semiconductor deviceof this embodiment, the potential of the semiconductor film 219 (inother words, the potential of the capacitor line 115) is constantlylower than the potential of the pixel electrode 121 by greater than orequal to the threshold voltage (Vth) of the capacitor 205 (MOScapacitor) in a period when the capacitor 205 is operated, as in theoperation method of the capacitor 105 in Embodiment 1. However, in thecapacitor 205, the semiconductor film 219 serving as one electrode isn-type and has high conductivity, so that the threshold voltage isshifted in the negative direction. The potential of the semiconductorfilm 219 (in other words, the potential of the capacitor line 115) canbe raised in accordance with the shift amount of the threshold voltageof the capacitor 205 in the negative direction, from the lowestpotential of the pixel electrode 121. Therefore, in the case where thethreshold voltage of the capacitor 205 is a larger negative value, thepotential of the capacitor line 115 can be higher than the potential ofthe pixel electrode 121.

When the semiconductor film 219 serving as one electrode of thecapacitor 205 is n-type and has high conductivity as in this embodiment,the threshold voltage can be shifted in the negative direction, so thatthe range of the potential needed for operating the capacitor 205 can bemade large as compared with the case of the capacitor 105 inEmbodiment 1. Thus, in this embodiment, the capacitor 205 can beconstantly operated with stability in an operation period of thecapacitor 205, which is preferable.

Further, since the semiconductor film 219 included in the capacitor 205is n-type and has high conductivity, enough charge capacity can beobtained even when the plane area of the capacitor 205 is reduced. Anoxide semiconductor included in the semiconductor film 219 transmits 80%to 90% of visible light; thus, when the area of the semiconductor film219 is reduced and a region where the semiconductor film 219 is notformed is provided in the pixel, the transmissivity with respect tolight emitted from a light source such as a backlight can be increased.

<Manufacturing Method of Semiconductor Device>

Next, a manufacturing method of an element portion provided over thesubstrate 102 described in this embodiment is described with referenceto FIGS. 11A and 11B and FIGS. 12A and 12B.

First, the scan line 107, the capacitor line 115, and the gate electrode627 are formed over the substrate 102. An insulating film which isprocessed into the gate insulating film 127 is formed over the substrate102, the scan line 107, the capacitor line 115, and the gate electrode627. The semiconductor film 111, the semiconductor film 119, and thesemiconductor film 631 are formed over the insulating film. The opening123 reaching the capacitor line 115 is formed in the insulating film toform the gate insulating film 127 and then the signal line 109, theconductive films 113 and 125, the source electrode 629, and the drainelectrode 633 are formed. The insulating film 128 is formed over thegate insulating film 127, the semiconductor films 111, 119, and 631, thesignal line 109, the conductive films 113 and 125, the source electrode629, and the drain electrode 633, and the insulating film 130 is formedover the insulating film 128 (see FIG. 11A). Note that the above stepscan be performed with reference to Embodiment 1.

Next, a mask is formed over a region of the insulating film 130, whichoverlaps with at least the semiconductor film 119. Processing isperformed to form an insulating film 228 and an insulating film 230 withthe use of the mask and expose the semiconductor film 119 (see FIG.11B). As the mask, a resist mask formed through a photolithographyprocess can be used, and the processing can be performed by one or bothof dry etching and wet etching.

Next, an insulating film 233 is formed over the exposed region of thesemiconductor film 119 and the insulating film 230, and an insulatingfilm 236 is formed over the insulating film 233 (see FIG. 12A). Theinsulating film 233 is similar to the insulating film 133 described inEmbodiment 1. The insulating film 236 is similar to the insulating film136 described in Embodiment 1. Heat treatment may be performed while theinsulating film 233 is in contact with the semiconductor film 119, forexample, after formation of the insulating film 233 and the insulatingfilm 236. The above steps can also be performed with reference toEmbodiment 1.

When the insulating film 233 is formed using a nitride insulating filmby a plasma CVD method or a sputtering method, the semiconductor film119 is exposed to plasma and oxygen vacancies are generated in thesemiconductor film 119. Moreover, when the semiconductor film 119 is incontact with the insulating film 233 formed using a nitride insulatingfilm, nitrogen and/or hydrogen are/is transferred from the insulatingfilm 233 to the semiconductor film 119. Due to entry of hydrogencontained in the insulating film 233 into an oxygen vacancy, an electronserving as a carrier is generated. Alternatively, when the insulatingfilm 232 is formed using a nitride insulating film and is subjected toheat treatment while it is in contact with the semiconductor film 119,nitrogen and/or hydrogen contained in the nitride insulating film can betransferred to the semiconductor film 119. Due to entry of hydrogencontained in the insulating film 233 into an oxygen vacancy, an electronserving as a carrier is generated. Accordingly, the semiconductor film119 has increased conductivity to be the n-type semiconductor film 219.Further, the semiconductor film 219 becomes a light-transmittingconductive film which includes a metal oxide film having conductorcharacteristics. Note that the conductivity of the semiconductor film219 is higher than that of the semiconductor film 111.

Next, the opening 117 reaching the conductive film 113 is formed in theinsulating films 228, 230, 233, and 236 to form the insulating films229, 231, 232, and 237 (see FIG. 9). Then, the pixel electrode 221 incontact with the conductive film 113 through the opening 117 is formed(see FIG. 12B). The above steps can be performed with reference toEmbodiment 1.

Through the above process, the semiconductor device in this embodimentcan be fabricated.

Modification Example

In the semiconductor device that is an embodiment of the presentinvention, the structure of the capacitor can be changed as appropriate.A specific example of the structure is described with reference to FIG.13. Here, only a capacitor 245 which is different from the capacitor 105described with reference to FIG. 9 and FIG. 10 is described.

In order that the semiconductor film 219 be n-type and have higherconductivity, a gate insulating film 227 has a layered structure of aninsulating film 225 formed of a nitride insulating film and aninsulating film 226 formed of an oxide insulating film and only theinsulating film 225 which is a nitride insulating film is provided in aregion where at least the semiconductor film 219 is provided. With sucha structure, the nitride insulating film which is the insulating film225 is in contact with the bottom surface of the semiconductor film 219,so that the semiconductor film 219 can be n-type and have higherconductivity. In this case, a dielectric film of the capacitor 245includes the insulating films 129, 131, 132, and 137. As the insulatingfilms 225 and 226, the insulating films which can be used as the gateinsulating film 127 can be used as appropriate, and the insulating film225 may be formed using an insulating film similar to the insulatingfilm 132. Further, to obtain this structure, the insulating film 226 isprocessed as appropriate with reference to Embodiment 1. A step ofetching the insulating films 129 and 131 is not performed with thestructure illustrated in FIG. 13; therefore, a reduction in thickness ofthe semiconductor film 219 can be prevented; thus, the yield isincreased as compared with the semiconductor device illustrated in FIG.9 and FIG. 10.

In the structure illustrated in FIG. 13, the top surface of thesemiconductor film 219 may be in contact with the insulating film 132.That is, regions of the insulating films 129 and 131 in FIG. 13 whichare in contact with the semiconductor film 219 may be removed. In thatcase, the insulating films 132 and 137 serve as the dielectric film ofthe capacitor 245. When the top and bottom surfaces of the semiconductorfilm 219 are in contact with the nitride insulating film, thesemiconductor film 219 can have a higher conductivity to be n-type moreefficiently and sufficiently than the semiconductor film 219 only onesurface of which is in contact with the nitride insulating film.

As described above, the use of the semiconductor film formed in the sameformation process as the semiconductor film included in the transistorfor one electrode of the capacitor allows fabrication of a semiconductordevice including the capacitor whose charge capacity is increased whilethe aperture ratio is improved to typically 55% or more, preferably 60%or more. As a result, the semiconductor device can have an excellentdisplay quality.

Further, oxygen vacancies and impurities such as hydrogen in thesemiconductor film formed using an oxide semiconductor and included inthe transistor are reduced, so that the semiconductor device that is anembodiment of the present invention has favorable electricalcharacteristics.

Note that the structure and the like described in this embodiment can beused as appropriate in combination with any of the structures and themodification examples in the other embodiments.

Embodiment 3

In this embodiment, a semiconductor device that is an embodiment of thepresent invention, which has a structure different from those describedin the above embodiment, is described with reference to drawings. Notethat in this embodiment, a liquid crystal display device is described asan example of the semiconductor device that is an embodiment of thepresent invention. In the semiconductor device described in thisembodiment, the structure of a semiconductor film included in acapacitor is different from that of the capacitor in the aboveembodiment. The above embodiment can be referred to for components inthe semiconductor device in this embodiment, which are similar to thoseof the semiconductor device in the above embodiment.

<Structure of Semiconductor Device>

Next, a specific example of the structure of a pixel 301 provided in apixel portion of the liquid crystal display device described in thisembodiment is described. FIG. 14 is a top view of the pixel 301. Thepixel 301 illustrated in FIG. 14 includes a capacitor 305, and thecapacitor 305 is provided in a region of the pixel 301, which issurrounded by the capacitor line 115 and the signal line 109. Thecapacitor 305 is electrically connected to the capacitor line 115through the conductive film 125 provided in and over the opening 123.The capacitor 305 is formed using an oxide semiconductor, and includes asemiconductor film 319 which has a higher conductivity than thesemiconductor film 111, the pixel electrode 121, and, as a dielectricfilm, the insulating films (not illustrated in FIG. 14) which isprovided over the transistor 103. The semiconductor film 319, the pixelelectrode 121, and the dielectric film transmit light; thus, thecapacitor 305 transmits light.

The conductivity of the semiconductor film 319 is higher than or equalto 10 S/cm and lower than or equal to 1000 S/cm, preferably higher thanor equal to 100 S/cm and lower than or equal to 1000 S/cm.

As described above, the semiconductor film 319 transmits light. That isto say, the capacitor 305 can be formed large (in a large area) in thepixel 101. Therefore, the semiconductor device can have charge capacityincreased while improving the aperture ratio to typically 55% or more,preferably 60% or more. Accordingly, the semiconductor device can haveexcellent display quality. Further, since the semiconductor film 319included in the capacitor 305 is n-type and has high conductivity, thesemiconductor film 319 can be considered as a conductive film. Further,since the semiconductor film 319 included in the capacitor 305 has highconductivity, enough charge capacity can be obtained even when the planearea of the capacitor 305 is reduced. An oxide semiconductor included inthe semiconductor film 319 transmits 80 to 90% of light; thus, when thearea of the semiconductor film 319 is reduced and a region where thesemiconductor film 319 is not formed is provided in the pixel, thetransmissivity with respect to light emitted from a light source such asa backlight can be increased.

Next, FIG. 15 illustrates a cross-sectional view of a transistorprovided in the scan line driver circuit 104 (see FIG. 1A) andcross-sectional views taken along dashed-dotted line A1-A2,dashed-dotted line B1-B2, and dashed-dotted line C1-C2 in FIG. 14. Here,a top view of the scan line driver circuit 104 is omitted, and across-sectional view of the scan line driver circuit 104 taken alongdashed-dotted line D1-D2 is illustrated. The cross-sectional view of thetransistor provided in the scan line driver circuit 104 is illustratedhere, and the transistor can be provided in the signal line drivercircuit 106.

The cross-sectional structure of the pixel 301 is as follows. Over thesubstrate 102, the scan line 107 of the transistor 103 is provided. Thegate insulating film 127 is provided over the scan line 107. Thesemiconductor film 111 is provided over a region of the gate insulatingfilm 127 which overlaps with the scan line 107. The semiconductor film319 is provided over the gate insulating film 127. The signal line 109of the transistor 103 and the conductive film 113 including the drainelectrode of the transistor 103 are provided over the semiconductor film111 and the gate insulating film 127. In addition, the capacitor line115 is provided over the gate insulating film 127 and the semiconductorfilm 319. The insulating films 129, 131, 132, and 137 which each serveas a protective insulating film of the transistor 103 is provided overthe gate insulating film 127, the signal line 109, the semiconductorfilm 111, the conductive film 113, the semiconductor film 319, and thecapacitor line 115. The opening 117 reaching the conductive film 113 isprovided in the insulating films 129, 131, 132, and 137 and the pixelelectrode 121 is provided in and over the opening 117 and over theinsulating film 132.

In the capacitor 305 in this structure, the semiconductor film 319 whichis n-type and has a higher conductivity than the semiconductor film 111serves as one of a pair of electrodes, the pixel electrode 121 serves asthe other of the pair of electrodes, and the insulating films 129, 131,132, and 137 serve as a dielectric film provided between the pair ofelectrodes.

For the semiconductor film 319, an oxide semiconductor that can be usedfor the semiconductor film 111 can be used. The semiconductor film 319can be formed concurrently with the semiconductor film 111 and thuscontains a metal element of an oxide semiconductor included in thesemiconductor film 111. Further, the semiconductor film 319 preferablyhas a higher conductivity than the semiconductor film 111 and thuspreferably contains an element (dopant) which increases theconductivity. Specifically, the semiconductor film 319 contains one ormore selected from boron, nitrogen, fluorine, aluminum, phosphorus,arsenic, indium, tin, antimony, and a rare gas element as the dopant.The concentration of a dopant contained in the semiconductor film 319 ispreferably greater than or equal to 1×10¹⁹ atoms/cm³ and less than orequal to 1×10²² atoms/cm³, in which case the conductivity of thesemiconductor film 319 can be greater than or equal to 10 S/cm and lessthan or equal to 1000 S/cm, preferably greater than or equal to 100 S/cmand less than or equal to 1000 S/cm, so that the semiconductor film 319can sufficiently serve as one electrode of the capacitor 305. Thesemiconductor film 319 has a region with a higher conductivity than thatof the semiconductor film 111. In this structure, at least a region ofthe semiconductor film 319, which is in contact with the insulating film132, has a higher conductivity than a region of the semiconductor film111, which is in contact with the insulating film 129. Further, thesemiconductor film 319 is n-type and has a high conductivity because ofincluding the above element (dopant); therefore, the semiconductor film319 can be called a conductive film.

<Manufacturing Method of Semiconductor Device>

Next, a manufacturing method of an element portion provided over thesubstrate 102 described in this embodiment is described with referenceto FIGS. 16A and 16B and FIGS. 17A and 17B.

First, the scan line 107, the capacitor line 115, and the gate electrode627 are formed over the substrate 102. The insulating film 126 which isprocessed into the gate insulating film 127 is formed over the substrate102, the scan line 107, the capacitor line 115, and the gate electrode627. The semiconductor films 111 and 119 are formed over the insulatingfilm 126. The semiconductor film 631 is provided in a region overlappingwith the gate electrode 627 (see FIG. 16A). Note that the above stepscan be performed with reference to Embodiment 1.

Next, a dopant is added to the semiconductor film 119 to form thesemiconductor film 319, the opening 123 reaching the capacitor line 115is formed in the insulating film 126 to form the gate insulating film127, and then the signal line 109 of the transistor 103, the conductivefilm 113 including the drain electrode of the transistor 103, and theconductive film 125 which electrically connects the semiconductor film319 and the capacitor line 115 are formed. Further, the source electrode629 and the drain electrode 633 are formed (see FIG. 16B).

A method of adding a dopant to the semiconductor film 119 is as follows:a mask is provided in a region except the semiconductor film 119 and oneor more dopants selected from boron, nitrogen, fluorine, aluminum,phosphorus, arsenic, indium, tin, antimony, and a rare gas element isadded to the semiconductor film 119 by an ion implantation method, anion doping method, or the like. Alternatively, the semiconductor film119 may be exposed to plasma containing the dopant to add the dopant tothe semiconductor film 119, instead of employing an ion implantationmethod or an ion doping method. Note that heat treatment may beperformed after the dopant is added to the semiconductor film 119. Theheat treatment can be performed as appropriate with reference to thedetails of the heat treatment for dehydration or dehydrogenation of thesemiconductor films 111 and 119.

The step of adding the dopant may be performed after formation of thesignal line 109, the conductive films 113 and 125, the source electrode629, and the drain electrode 633, in which case the dopant is not addedto regions of the semiconductor film 319 which are in contact with thesignal line 109, the conductive film 113, and the conductive film 125.

Next, the insulating film 128 is formed over the semiconductor films111, 319, and 631, the signal line 109, the conductive films 113 and125, the source electrode 629, the drain electrode 633, and the gateinsulating film 127. The insulating film 130 is formed over theinsulating film 128, and the insulating film 133 is formed over theinsulating film 130. The insulating film 136 is formed over theinsulating film 133 (see FIG. 17A). The above steps can be performedwith reference to Embodiment 1.

Next, the opening 117 (see FIG. 14) reaching the conductive film 113 isformed in regions of the insulating films 128, 130, 133, and 136 whichoverlap with the conductive film 113, and the insulating films 129, 131,132, and 137 are formed. The pixel electrode 121 in contact with theconductive film 113 through the opening 117 is formed (see FIG. 17B).The above steps can be performed with reference to Embodiment 1.

Through the above process, the semiconductor device in this embodimentcan be fabricated.

Accordingly, the use of the semiconductor film formed in the sameformation process as the semiconductor film included in the transistor,for one electrode of the capacitor, allows fabrication of asemiconductor device including the capacitor whose charge capacity isincreased while the aperture ratio is improved. As a result, thesemiconductor device can have an excellent display quality.

Further, oxygen vacancies and impurities such as hydrogen in thesemiconductor film formed using an oxide semiconductor and included inthe transistor are reduced, so that the semiconductor device that is anembodiment of the present invention has favorable electricalcharacteristics.

Note that the structure and the like described in this embodiment can beused as appropriate in combination with any of the structures and themodification examples in the other embodiments.

Embodiment 4

In this embodiment, one embodiment which can be applied to an oxidesemiconductor included in a semiconductor film in the transistor and thecapacitor included in the semiconductor device described in the aboveembodiment is described.

The oxide semiconductor film is preferably formed using any of anamorphous oxide semiconductor, a single crystal oxide semiconductor, apolycrystalline oxide semiconductor, and an oxide semiconductorincluding a crystalline portion (a c-axis aligned crystalline oxidesemiconductor (CAAC-OS)). The CAAC-OS film is one of oxide semiconductorfilms including a plurality of crystal parts, and most of each crystalpart fits inside a cube whose one side is less than 100 nm. Thus, thereis a case where a crystal part included in the CAAC-OS film fits a cubewhose one side is less than 10 nm, less than 5 nm, or less than 3 nm.The density of defect states of the CAAC-OS film is lower than that ofthe microcrystalline oxide semiconductor film. The CAAC-OS film isdescribed in detail below.

In a transmission electron microscope (TEM) image of the CAAC-OS film, aboundary between crystal parts, that is, a grain boundary is not clearlyobserved. Thus, in the CAAC-OS film, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

According to the TEM image of the CAAC-OS film observed in a directionsubstantially parallel to a sample surface (cross-sectional TEM image),metal atoms are arranged in a layered manner in the crystal parts. Eachmetal atom layer has a morphology reflected by a surface over which theCAAC-OS film is formed (hereinafter, a surface over which the CAAC-OSfilm is formed is referred to as a formation surface) or a top surfaceof the CAAC-OS film, and is arranged in parallel to the formationsurface or the top surface of the CAAC-OS film.

On the other hand, according to the TEM image of the CAAC-OS filmobserved in a direction substantially perpendicular to the samplesurface (plan TEM image), metal atoms are arranged in a triangular orhexagonal configuration in the crystal parts. However, there is noregularity of arrangement of metal atoms between different crystalparts.

From the results of the cross-sectional TEM image and the plan TEMimage, alignment is found in the crystal parts in the CAAC-OS film.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ) is around 31°.This peak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS film.

On the other hand, when the CAAC-OS film is analyzed by an in-planemethod in which an X-ray enters a sample in a direction substantiallyperpendicular to the c-axis, a peak appears frequently when 2θ is around56°. This peak is derived from the (110) plane of the InGaZnO₄ crystal.Here, analysis (φ scan) is performed under conditions where the sampleis rotated around a normal vector of a sample surface as an axis (φaxis) with 2θ fixed at around 56°. In the case where the sample is asingle-crystal oxide semiconductor film of InGaZnO₄, six peaks appear.The six peaks are derived from crystal planes equivalent to the (110)plane. On the other hand, in the case of a CAAC-OS film, a peak is notclearly observed even when φ scan is performed with 2θ fixed at around56°.

According to the above results, in the CAAC-OS film having c-axisalignment, while the directions of a-axes and b-axes are differentbetween crystal parts, the c-axes are aligned in a direction parallel toa normal vector of a formation surface or a normal vector of a topsurface. Thus, each metal atom layer arranged in a layered mannerobserved in the cross-sectional TEM image corresponds to a planeparallel to the a-b plane of the crystal.

Note that the crystal part is formed concurrently with deposition of theCAAC-OS film or is formed through crystallization treatment such as heattreatment. As described above, the c-axis of the crystal is aligned witha direction parallel to a normal vector of a formation surface or anormal vector of a top surface. Thus, for example, in the case where ashape of the CAAC-OS film is changed by etching or the like, the c-axismight not be necessarily parallel to a normal vector of a formationsurface or a normal vector of a top surface of the CAAC-OS film.

Further, the degree of crystallinity in the CAAC-OS film is notnecessarily uniform. For example, in the case where crystal growthleading to the CAAC-OS film occurs from the vicinity of the top surfaceof the film, the degree of the crystallinity in the vicinity of the topsurface is higher than that in the vicinity of the formation surface insome cases. Further, when an impurity is added to the CAAC-OS film, thecrystallinity in a region to which the impurity is added is changed, andthe degree of crystallinity in the CAAC-OS film varies depending onregions.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed byan out-of-plane method, a peak of 2θ may also be observed at around 36°,in addition to the peak of 2θ at around 31°. The peak of 2θ at around36° indicates that a crystal having no c-axis alignment is included inpart of the CAAC-OS film. It is preferable that in the CAAC-OS film, apeak of 2θ appears at around 31° and a peak of 2θ do not appear ataround 36°.

For example, there are three methods for forming a CAAC-OS film.

The first method is to form an oxide semiconductor film at a temperaturehigher than or equal to 100° C. and lower than or equal to 450° C.,whereby crystal parts in which the c-axes are aligned in the directionparallel to a normal vector of a surface on which the oxidesemiconductor film is formed or a normal vector of a surface of theoxide semiconductor film are formed in the oxide semiconductor film.

The second method is to form an oxide semiconductor film with a smallthickness and then heat it at a temperature higher than or equal to 200°C. and lower than or equal to 700° C., whereby crystal parts in whichthe c-axes are aligned in the direction parallel to a normal vector of asurface on which the oxide semiconductor film is formed or a normalvector of a surface of the oxide semiconductor film are formed in theoxide semiconductor film.

The third method is to form a first oxide semiconductor film with asmall thickness, then heat it at a temperature higher than or equal to200° C. and lower than or equal to 700° C., and form a second oxidesemiconductor film, whereby crystal parts in which the c-axes arealigned in the direction parallel to a normal vector of a surface onwhich the oxide semiconductor film is formed or a normal vector of asurface of the oxide semiconductor film are formed in the oxidesemiconductor film.

In a transistor using the CAAC-OS film for an oxide semiconductor film,change in the electrical characteristics of the transistor due toirradiation with visible light or ultraviolet light is small. Thus, thetransistor using the CAAC-OS film as the oxide semiconductor film hashigh reliability.

For example, it is preferable that the CAAC-OS film is formed by asputtering method with a polycrystalline oxide semiconductor sputteringtarget. When ions collide with the sputtering target, a crystal regionincluded in the sputtering target may be separated from the target alongan a-b plane; in other words, a sputtered particle having a planeparallel to an a-b plane (flat-plate-like sputtered particle orpellet-like sputtered particle) may flake off from the sputteringtarget. In that case, the flat-plate-like sputtered particle or thepellet-like sputtered particle reaches a surface on which the CAAC-OSfilm is formed while maintaining its crystal state, whereby the CAAC-OSfilm can be deposited.

For the deposition of the CAAC-OS film, the following conditions arepreferably employed.

By reducing the amount of impurities entering the CAAC-OS film duringthe deposition, the crystal state can be prevented from being broken bythe impurities. For example, the concentration of impurities (e.g.,hydrogen, water, carbon dioxide, or nitrogen) which exist in thedeposition chamber may be reduced. Furthermore, the concentration ofimpurities in a deposition gas may be reduced. Specifically, adeposition gas whose dew point is lower than or equal to −80° C.,preferably lower than or equal to −100° C. is used.

By increasing the heating temperature of the surface on which theCAAC-OS film is formed (e.g., the substrate heating temperature) duringthe deposition, migration of a sputtered particle is likely to occurafter the sputtered particle reaches the surface on which the CAAC-OSfilm is formed. Specifically, the temperature of the surface on whichthe CAAC-OS film is formed during the deposition is higher than or equalto 100° C. and lower than or equal to 740° C., preferably higher than orequal to 150° C. and lower than or equal to 500° C. By increasing thetemperature of the surface on which the CAAC-OS film is formed duringthe deposition, when the flat-plate-like or pellet-like sputteredparticle reaches the surface on which the CAAC-OS film is formed,migration occurs on the surface on which the CAAC-OS film is formed, sothat a flat plane of the sputtered particle is attached to the surfaceon which the CAAC-OS film is formed.

Furthermore, it is preferable that the proportion of oxygen in thedeposition gas be increased and the power be optimized in order toreduce plasma damage at the deposition. The proportion of oxygen in thedeposition gas is higher than or equal to 30 vol %, preferably 100 vol%.

As an example of the sputtering target, an In—Ga—Zn—O compound target isdescribed below.

The polycrystalline In—Ga—Zn—O compound target is made by mixing InO_(X)powder, GaO_(Y) powder, and ZnO_(Z) powder in a predetermined molarratio, applying pressure, and performing heat treatment at a temperaturehigher than or equal to 1000° C. and lower than or equal to 1500° C.This pressure treatment may be performed while cooling is performed ormay be performed while heating is performed. Note that X, Y, and Z areeach a given positive number. Here, the predetermined molar ratio ofInO_(X) powder to GaO_(Y) powder and ZnO_(Z) powder is, for example,2:2:1, 8:4:3, 3:1:1, 1:1:1, 4:2:3, or 3:1:2. The kinds of powder and themolar ratio for mixing powder may be determined as appropriate dependingon the desired sputtering target.

Further, the oxide semiconductor film may have a structure in which aplurality of oxide semiconductor films are stacked. For example, theoxide semiconductor film may have a stacked-layer structure of a firstoxide semiconductor film and a second oxide semiconductor film which areformed using metal oxides with different atomic ratios. For example, thefirst oxide semiconductor film may be formed using one of an oxidecontaining two kinds of metals, an oxide containing three kinds ofmetals, and an oxide containing four kinds of metals, and the secondoxide semiconductor film may be formed using one of the above which isdifferent from the one used for the first oxide semiconductor film.

Alternatively, the oxide semiconductor film may have a two-layerstructure of the first oxide semiconductor film and the second oxidesemiconductor film, in which the constituent elements thereof are madethe same and the atomic ratios of the constituent elements of the firstoxide semiconductor film and the second oxide semiconductor film aremade different. For example, the first oxide semiconductor film maycontain In, Ga, and Zn at an atomic ratio of 3:1:2, and the second oxidesemiconductor film may contain In, Ga, and Zn at an atomic ratio of1:1:1. Alternatively, the first oxide semiconductor film may contain In,Ga, and Zn at an atomic ratio of 2:1:3, and the second oxidesemiconductor film may contain In, Ga, and Zn at an atomic ratio of1:3:2. Note that a proportion of each atom in the atomic ratio of theoxide semiconductor film varies within a range of ±20% as an error.

At this time, in one of the first oxide semiconductor film and thesecond oxide semiconductor film, which is closer to the gate electrode(on the channel side), the atomic ratio of In to Ga preferably satisfiesthe relation In≧Ga. In the other oxide semiconductor film, which isfarther from the gate electrode (on the back channel side), the atomicratio of In to Ga preferably satisfies the relation In<Ga. With astacked-layer structure of these oxide semiconductor films, a transistorhaving high field-effect mobility can be formed. On the other hand, theatomic ratio of In to Ga in the oxide semiconductor film closer to thegate electrode (the oxide semiconductor film on the channel side)satisfies the relation In<Ga and the atomic ratio of In to Ga in theoxide semiconductor film on the back channel side satisfies the relationIn≧Ga, whereby the amount of change of the threshold voltage of atransistor due to change over time or a reliability test can be reduced.

The first oxide semiconductor film containing In, Ga, and Zn at anatomic ratio of 1:3:2 can be formed by a sputtering method using anoxide target with an atomic ratio of 1:3:2 under the conditions wherethe substrate temperature is room temperature and a sputtering gas isargon or a mixed gas of argon and oxygen. The second oxide semiconductorfilm containing In, Ga, and Zn at an atomic ratio of 3:1:2 can be formedby a sputtering method using an oxide target with an atomic ratio of3:1:2 similarly to the first oxide semiconductor film.

Further, the oxide semiconductor film may have a three-layer structureof a first oxide semiconductor film, a second oxide semiconductor film,and a third oxide semiconductor film, in which the constituent elementsthereof are made the same and the atomic ratios of the constituentelements of the first oxide semiconductor film, the second oxidesemiconductor film, and the third oxide semiconductor film are madedifferent. The case where the oxide semiconductor film has a three-layerstructure is described with reference to FIG. 18.

In a transistor illustrated in FIG. 18, a first oxide semiconductor film199 a, a second oxide semiconductor film 199 b, and a third oxidesemiconductor film 199 c are stacked in this order from the gateinsulating film 127 side. As a material of the first oxide semiconductorfilm 199 a and the third oxide semiconductor film 199 c, a materialrepresented by InM1_(x)Zn_(y)O_(z) (x≧1, y>1, z>0, M1=Ga, Hf, or thelike) is used. Note that in the case where a material of each of thefirst oxide semiconductor film 199 a and the third oxide semiconductorfilm 199 c contains Ga, a material containing a high proportion of Ga,specifically, a material which can be represented by InM1_(x)Zn_(y)O_(z)where x is larger than 10 is unsuitable because powder might begenerated in deposition.

As a material of the second oxide semiconductor film 199 b, a materialwhich can be represented by InM2_(x)Zn_(y)O_(z) (x≧1, y≧x, z>0, M2=Ga,Sn, or the like) is used.

Materials of the first oxide semiconductor film 199 a, the second oxidesemiconductor film 199 b, and the third oxide semiconductor film 199 care selected as appropriate so that a well structure is formed in whichthe conduction band of the second oxide semiconductor film 199 b isdeeper from the vacuum level than the conduction bands of the firstoxide semiconductor film 199 a and the third oxide semiconductor film199 c.

Note that silicon and carbon, which are Group 14 elements, are donorsupply sources in an oxide semiconductor film. Therefore, silicon orcarbon contained in an oxide semiconductor film makes it n-type. Thus,the concentration of silicon contained in oxide semiconductor films andthe concentration of carbon contained in oxide semiconductor films areeach less than or equal to 3×10¹⁸/cm³, preferably less than or equal to3×10¹⁷/cm³. It is particularly preferable to employ a structure wherethe first oxide semiconductor film 199 a and the third oxidesemiconductor film 199 c sandwich or surround the second oxidesemiconductor film 199 b serving as a carrier path so that a largenumber of Group 14 elements do not enter the second oxide semiconductorfilm 199 b. That is, the first oxide semiconductor film 199 a and thethird oxide semiconductor film 199 c can also be called barrier filmswhich prevent Group 14 elements such as silicon and carbon from enteringthe second oxide semiconductor film 199 b.

For example, the first oxide semiconductor film 199 a may contain In,Ga, and Zn at an atomic ratio of 1:3:2, the second oxide semiconductorfilm 199 b may contain In, Ga, and Zn at an atomic ratio of 3:1:2, andthe third oxide semiconductor film 199 c may contain In, Ga, and Zn atan atomic ratio of 1:1:1. Note that the third oxide semiconductor film199 c can be formed by a sputtering method using an oxide targetcontaining In, Ga, and Zn at an atomic ratio of 1:1:1.

Alternatively, a three-layer structure may be employed in which thefirst oxide semiconductor film 199 a contains In, Ga, and Zn at anatomic ratio of 1:3:2, the second oxide semiconductor film 199 bcontains In, Ga, and Zn at an atomic ratio of 1:1:1 or 1:3:2, and thethird oxide semiconductor film 199 c contains In, Ga, and Zn at anatomic ratio of 1:3:2.

Since the constituent elements of the first oxide semiconductor film 199a, the second oxide semiconductor film 199 b, and the third oxidesemiconductor film 199 c are the same, the second oxide semiconductorfilm 199 b has fewer defect states (trap levels) at the interface withthe first oxide semiconductor film 199 a. Specifically, the defectstates (trap levels) are fewer than those at the interface between thegate insulating film 127 and the first oxide semiconductor film 199 a.For this reason, when the oxide semiconductor films are stacked in theabove manner, the amount of change of the threshold voltage of atransistor due to change over time or a reliability test can be reduced.

Further, when materials of the first oxide semiconductor film 199 a, thesecond oxide semiconductor film 199 b, and the third oxide semiconductorfilm 199 c are selected as appropriate so that a well structure isformed in which the conduction band of the second oxide semiconductorfilm 199 b is deeper from the vacuum level than the conduction bands ofthe first oxide semiconductor film 199 a and the third oxidesemiconductor film 199 c, the field-effect mobility of the transistorcan be increased and the amount of change of the threshold voltage ofthe transistor due to change over time or a reliability test can bereduced.

Further, the first oxide semiconductor film 199 a, the second oxidesemiconductor film 199 b, and the third oxide semiconductor film 199 cmay be formed using an oxide semiconductors having differentcrystallinity. That is, the oxide semiconductor film may be formed usinga combination of any of a single crystal oxide semiconductor, apolycrystalline oxide semiconductor, an amorphous oxide semiconductor,and a CAAC-OS, as appropriate. When an amorphous oxide semiconductor isapplied to any one of the first oxide semiconductor film 199 a, thesecond oxide semiconductor film 199 b, and the third oxide semiconductorfilm 199 c, internal stress or external stress of the oxidesemiconductor film can be relieved, fluctuation in characteristics ofthe transistors can be reduced, and the amount of change of thethreshold voltage of the transistor due to change over time or areliability test can be reduced.

At least the second oxide semiconductor film 199 b, which can serve as achannel formation region, is preferably a CAAC-OS film. An oxidesemiconductor film on the back channel side, in this embodiment, thethird oxide semiconductor film 199 c is preferably an amorphous oxidesemiconductor film or a CAAC-OS film. With such a structure, the amountof change of the threshold voltage of a transistor due to change overtime or a reliability test can be reduced.

Note that the structure and the like described in this embodiment can beused as appropriate in combination with any of the structures and thelike in the other embodiments.

Embodiment 5

A semiconductor device (also referred to as a display device) having adisplay function can be fabricated using the transistor and thecapacitor examples of which are shown in the above embodiments.Moreover, some or all of driver circuits which include the transistorcan be formed over a substrate where a pixel portion is formed, wherebya system-on-panel can be obtained. In this embodiment, an example of adisplay device using the transistor examples of which are shown in theabove embodiments is described with reference to FIGS. 19A to 19C, FIG.20, and FIGS. 21A to 21C. FIG. 20 are cross-sectional views illustratingcross-sectional structures taken along dashed-dotted line M-N in FIG.19B. Note that in FIG. 20, only part of the structure of a pixel portionis illustrated.

In FIG. 19A, a sealant 905 is provided so as to surround a pixel portion902 provided over a first substrate 901, and the pixel portion 902 issealed with a second substrate 906. In FIG. 19A, a signal line drivercircuit 903 and a scan line driver circuit 904 are each formed using asingle crystal semiconductor or a polycrystalline semiconductor over asubstrate prepared separately, and mounted in a region different fromthe region surrounded by the sealant 905 over the first substrate 901.Further, various signals and potentials are supplied to the signal linedriver circuit 903, the scan line driver circuit 904, and the pixelportion 902 from flexible printed circuits (FPCs) 918 a and 918 b.

In FIGS. 19B and 19C, the sealant 905 is provided so as to surround thepixel portion 902 and the scan line driver circuit 904 which areprovided over the first substrate 901. The second substrate 906 isprovided over the pixel portion 902 and the scan line driver circuit904. Thus, the pixel portion 902 and the scan line driver circuit 904are sealed together with a display element by the first substrate 901,the sealant 905, and the second substrate 906. In FIGS. 19B and 19C, asignal line driver circuit 903 which is formed using a single crystalsemiconductor or a polycrystalline semiconductor over a substrateseparately prepared is mounted in a region different from the regionsurrounded by the sealant 905 over the first substrate 901. In FIGS. 19Band 19C, various signals and potentials are supplied to the signal linedriver circuit 903, the scan line driver circuit 904, and the pixelportion 902 from an FPC 918.

Although FIGS. 19B and 19C each illustrate an example in which thesignal line driver circuit 903 is formed separately and mounted on thefirst substrate 901, an embodiment of the present invention is notlimited to this structure. The scan line driver circuit may beseparately formed and then mounted, or only part of the signal linedriver circuit or part of the scan line driver circuit may be separatelyformed and then mounted.

Note that a connection method of a separately formed driver circuit isnot particularly limited, and a chip on glass (COG) method, a wirebonding method, a tape automated bonding (TAB) method, or the like canbe employed. FIG. 19A illustrates an example in which the signal linedriver circuit 903 and the scan line driver circuit 904 are mounted by aCOG method. FIG. 19B illustrates an example in which the signal linedriver circuit 903 is mounted by a COG method. FIG. 19C illustrates anexample in which the signal line driver circuit 903 is mounted by a TABmethod.

The display device includes in its category a panel in which a displayelement is sealed and a module in which an IC including a controller orthe like is mounted on the panel.

A display device in this specification refers to an image display deviceor a display device. The display device may serve as a light source(including a lighting device). Further, the display device also includesthe following modules in its category: a module to which a connectorsuch as an FPC or a TCP is attached; a module having a TCP at the tip ofwhich a printed wiring board is provided; and a module in which anintegrated circuit (IC) is directly mounted on a display element by aCOG method.

The pixel portion and the scan line driver circuit provided over thefirst substrate include a plurality of transistors and any of thetransistors which are described in the above embodiments can be applied.

As the display element provided in the display device, a liquid crystalelement (also referred to as a liquid crystal display element) or alight-emitting element (also referred to as a light-emitting displayelement) can be used. A light-emitting element includes, in its scope,an element whose luminance is controlled by current or voltage, andspecifically includes an inorganic electroluminescent (EL) element, anorganic EL element, and the like. Further, a display medium whosecontrast is changed by an electric effect, such as electronic ink, canbe used. An example of a liquid crystal display device using a liquidcrystal element as the display element is illustrated in FIG. 20.

The liquid crystal display device illustrated in FIG. 20 is a liquidcrystal display device of a vertical electric field mode. A liquidcrystal display device includes a connection terminal electrode 915 anda terminal electrode 916. The connection terminal electrode 915 and theterminal electrode 916 are electrically connected to a terminal includedin the FPC 918 through an anisotropic conductive agent 919.

The connection terminal electrode 915 is formed using the sameconductive film as a first electrode 930, and the terminal electrode 916is formed using the same conductive film as a source electrode and adrain electrode of each of a transistor 910 and a transistor 911.

Each of the pixel portion 902 and the scan line driver circuit 904 whichare provided over the first substrate 901 includes a plurality oftransistors. The transistor 910 included in the pixel portion 902 andthe transistor 911 included in the scan line driver circuit 904 areillustrated in FIG. 20. An insulating film 924 corresponding to theinsulating films 129, 131, and 132 described in Embodiment 1 is providedover the transistors 910 and 911. Further, the insulating film 934 isprovided over the insulating film 924 corresponding to the insulatingfilm 137 described in Embodiment 1. Note that an insulating film 923 isan insulating film serving as a base film.

In this embodiment, any of the transistors provided in the pixeldescribed in Embodiments 1 to 3 can be applied to the transistor 910.Further, any of the transistors described in Embodiments 1 to 3, whichis provided in the scan line driver circuit, can be applied to thetransistor 911. A capacitor 936 is formed using an oxide semiconductorfilm 927, the insulating films 924 and 934, and the first electrode 930.The oxide semiconductor film 927 is electrically connected to acapacitor line 929 through an electrode 928. The electrode 928 is formedusing the same material and steps as the source electrode and the drainelectrode of each of the transistors 910 and 911. The capacitor line 929is formed using the same material and steps as a gate electrode of eachof the transistors 910 and 911. Although the capacitor described inEmbodiment 1 is illustrated as the capacitor 936 here, any of thecapacitors in the other embodiments may be used as appropriate.

In the display panel, the transistor 910 included in the pixel portion902 is electrically connected to a display element. There is noparticular limitation on the kind of the display element as long asdisplay can be performed, and various kinds of display elements can beused.

A liquid crystal element 913 which is a display element includes thefirst electrode 930, a second electrode 931, and a liquid crystal layer908. Note that an insulating film 932 and an insulating film 933 whichserve as an alignment film are provided so that the liquid crystal layer908 is provided therebetween. The second electrode 931 is provided onthe second substrate 906 side. The second electrode 931 overlaps withthe first electrode 930 with the liquid crystal layer 908 providedtherebetween.

The first electrode and the second electrode (each of which are alsoreferred to as a pixel electrode, a common electrode, a counterelectrode, or the like) for applying voltage to the display element canhave light-transmitting properties or light-reflecting properties, whichdepends on the direction in which light is extracted, the position wherethe electrodes are provided, and the pattern structure of theelectrodes.

The first electrode 930 and the second electrode 931 can be formedusing, as appropriate, a material similar to that of the pixel electrode121 of Embodiment 1.

A spacer 935 is a columnar spacer obtained by selective etching of aninsulating film and is provided in order to control the distance betweenthe first electrode 930 and the second electrode 931 (a cell gap).Alternatively, a spherical spacer may be used.

In the case where a liquid crystal element is used as the displayelement, a thermotropic liquid crystal, a low-molecular liquid crystal,a high-molecular liquid crystal, a polymer dispersed liquid crystal, aferroelectric liquid crystal, an anti-ferroelectric liquid crystal, orthe like can be used. Some examples are given as a vertical alignmentmode.

Alternatively, liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used. A blue phase is one of liquidcrystal phases, which is generated just before a cholesteric phasechanges into an isotropic phase while temperature of cholesteric liquidcrystal is increased. Since the blue phase appears only in a narrowtemperature range, a liquid crystal composition in which a chiralmaterial is mixed is used for the liquid crystal layer in order toimprove the temperature range. Note that the alignment film is formedusing an organic resin containing hydrogen, water, or the like, whichmight degrade the electrical characteristics of the transistor in thesemiconductor device that is an embodiment of the present invention. Inview of the above, the use of liquid crystal which exhibits a blue phasefor the liquid crystal layer enables fabrication of the semiconductordevice that is an embodiment of the present invention without an organicresin, so that the semiconductor device can be highly reliable.

The first substrate 901 and the second substrate 906 are fixed in placeby a sealant 925. As the sealant 925, an organic resin such as athermosetting resin or a photocurable resin can be used. In addition,the sealant 925 is in contact with the insulating film 924. Note thatthe sealant 925 corresponds to the sealant 905 in FIGS. 19A to 19C.

The sealant 925 is provided over the insulating film 924. The insulatingfilm 934 is provided on the inner side of the sealant 925. The uppermostlayer of the insulating film 924 is a nitride insulating film, and cansuppress entry of impurities such as hydrogen and water from theoutside. In contrast, the insulating film 934 has a high moisturepermeability. Thus, the insulating film 934 is provided on the innerside of the sealant 925, and the inner side of the sealant 925 isprovided over the insulating film 924, so that entry of impurities suchas hydrogen and water from the outside can be suppressed, and thusvariations in the electrical characteristics of the transistors 910 and911 can be suppressed.

In the liquid crystal display device, a black matrix (a light-blockingfilm); an optical member (an optical substrate) such as a polarizingmember, a retardation member, or an anti-reflection member; and the likeare provided as appropriate. For example, circular polarization may beobtained by using a polarizing substrate and a retardation substrate. Inaddition, a backlight, a side light, or the like may be used as a lightsource.

Since the transistor is easily broken owing to static electricity or thelike, a protective circuit for protecting the driver circuit ispreferably provided. The protection circuit is preferably formed using anonlinear element.

FIGS. 21A to 21C illustrate an example of the liquid crystal displaydevice in FIG. 20 in which a common connection portion (pad portion) forbeing electrically connected to the second electrode 931 provided on thesubstrate 906 is formed over the substrate 901.

The common connection portion is provided in a position overlapping withthe sealant for bonding the substrate 901 and the substrate 906, and iselectrically connected to the second electrode 931 through conductiveparticles contained in the sealant. Alternatively, the common connectionportion is provided in a position not overlapping with the sealant(except for the pixel portion) and a paste including conductiveparticles is provided separately from the sealant so as to overlap withthe common connection portion, whereby the common connection portion iselectrically connected to the second electrode 931.

FIG. 21A is a cross-sectional view of the common connection portiontaken along line I-J in the top view in FIG. 21B.

A common potential line 975 is provided over a gate insulating film 922and is formed using the same material and through the same steps as asource electrode 971 or a drain electrode 973 of the transistor 910illustrated in FIGS. 21A to 21C.

Further, the common potential line 975 is covered with the insulatingfilms 924 and 934, and the insulating films 924 and 934 has a pluralityof openings at a position overlapping with the common potential line975. These openings are formed through the same steps as a contact holewhich connects the first electrode 930 and one of the source electrode971 and the drain electrode 973 of the transistor 910.

Further, the common potential line 975 is connected to a commonelectrode 977 through the openings. The common electrode 977 is providedover the insulating film 934 and is formed using the same material andthrough the same steps as the connection terminal electrode 915 and thefirst electrode 930 in the pixel portion.

In this manner, the common connection portion can be formed through thesame manufacturing process as the switching element in the pixel portion902.

The common electrode 977 is an electrode in contact with the conductiveparticles contained in the sealant, and is electrically connected to thesecond electrode 931 of the second substrate 906.

Alternatively, as illustrated in FIG. 21C, a common potential line 985may be formed using the same material and through the same steps as thegate electrode of the transistor 910.

In the common connection portion illustrated in FIG. 21C, the commonpotential line 985 is provided under the gate insulating films 922, 924,and 934; and the gate insulating films 922, 924, and 934 have aplurality of openings at a position overlapping with the commonpotential line 985. These openings are formed by etching the insulatingf films 924 and 934 through the same steps as a contact hole whichconnects the first electrode 930 and one of the source electrode 971 andthe drain electrode 973 of the transistor 910, and then by furtherselectively etching the gate insulating film 922.

Further, the common potential line 985 is connected to a commonelectrode 987 through the openings. The common electrode 987 is providedover the insulating film 924 and is formed using the same material andthrough the same steps as the connection terminal electrode 915 and thefirst electrode 930 in the pixel portion.

As described above, the use of the transistor and capacitor described inthe above embodiment allows fabrication of a semiconductor deviceincluding the capacitor whose charge capacity is increased whileimproving the aperture ratio. Accordingly, the semiconductor device canhave excellent display quality.

Further, oxygen vacancies and impurities such as hydrogen in thesemiconductor film formed using an oxide semiconductor included in thetransistor, which is a semiconductor film included in the transistor,are reduced, so that the semiconductor device that is an embodiment ofthe present invention has favorable electrical characteristics.

Note that the structures and the like described in this embodiment canbe combined as appropriate with any of the structures and the likedescribed in the other embodiments and example.

Embodiment 6

The semiconductor device that is an embodiment of the present inventioncan be applied to any of a variety of electronic devices (including gamemachines). Examples of electronic devices include television sets (alsoreferred to as televisions or television receivers), monitors ofcomputers, cameras such as digital cameras or digital video cameras,digital photo frames, mobile phones, portable game consoles, portableinformation terminals, audio reproducing devices, game machines (e.g.,pachinko machines or slot machines), housings of game machines, and thelike. Examples of such electronic devices are illustrated in FIGS. 22Ato 22C.

FIG. 22A illustrates a table 9000 having a display portion. In the table9000, a display portion 9003 is incorporated in a housing 9001 and animage can be displayed on the display portion 9003. Note that thehousing 9001 is supported by four leg portions 9002. Further, a powercord 9005 for supplying power is provided for the housing 9001.

The semiconductor device described in any of the above embodiments canbe used for the display portion 9003. Thus, the display quality of thedisplay portion 9003 can be improved.

The display portion 9003 has a touch-input function. When a user touchesdisplayed buttons 9004 which are displayed on the display portion 9003of the table 9000 with his/her finger or the like, the user can carryout operation of the screen and input of information. Further, when thetable 9000 may be made to communicate with home appliances or controlthe home appliances, the table 9000 may serve as a control device whichcontrols the home appliances by operation on the screen. For example,with the use of a semiconductor device having an image sensor function,the display portion 9003 can have a touch-input function.

Further, the screen of the display portion 9003 can be placedperpendicular to a floor with a hinge provided for the housing 9001;thus, the table 9000 can also be used as a television device. When atelevision device having a large screen is set in a small room, an openspace is reduced; however, when a display portion is incorporated in atable, a space in the room can be efficiently used.

FIG. 22B illustrates a television set 9100. In the television set 9100,a display portion 9103 is incorporated in a housing 9101 and an imagecan be displayed on the display portion 9103. Note that here, thehousing 9101 is supported by a stand 9105.

The television set 9100 can be operated with an operation switch of thehousing 9101 or a separate remote controller 9110. Channels and volumecan be controlled with an operation key 9109 of the remote controller9110 so that an image displayed on the display portion 9103 can becontrolled. Further, the remote controller 9110 may be provided with adisplay portion 9107 for displaying data output from the remotecontroller 9110.

The television set 9100 illustrated in FIG. 22B is provided with areceiver, a modem, and the like. With the use of the receiver, thetelevision set 9100 can receive general TV broadcasts. Moreover, whenthe television set 9100 is connected to a communication network with orwithout wires via the modem, one-way (from a sender to a receiver) ortwo-way (between a sender and a receiver or between receivers)information communication can be performed.

The semiconductor device described in any of the above embodiments canbe used for the display portions 9103 and 9107. Thus, the displayquality of the television set can be improved.

FIG. 22C illustrates a computer 9200, which includes a main body 9201, ahousing 9202, a display portion 9203, a keyboard 9204, an externalconnection port 9205, a pointing device 9206, and the like.

The semiconductor device described in any of the above embodiments canbe used for the display portion 9203. Thus, the display quality of thecomputer 9200 can be improved.

FIGS. 23A and 23B illustrate a foldable tablet terminal. In FIG. 23A,the tablet terminal is opened, and includes a housing 9630, a displayportion 9631 a, a display portion 9631 b, a display-mode switchingbutton 9034, a power button 9035, a power-saving-mode switching button9036, a clip 9033, and an operation button 9038.

The semiconductor device described in any of the above embodiments canbe used for the display portion 9631 a and the display portion 9631 b.Thus, the display quality of the tablet terminal can be improved.

Part of the display portion 9631 a can be a touch panel region 9632 a,and data can be input by touching operation keys 9638 that aredisplayed. Note that FIG. 23A shows, as an example, that half of thearea of the display portion 9631 a has only a display function, and theother half of the area has a touch panel function. However, thestructure of the display portion 9631 a is not limited to this, and allthe area of the display portion 9631 a may have a touch panel function.For example, all the area of the display portion 9631 a can displaykeyboard buttons and serve as a touch panel while the display portion9631 b can be used as a display screen.

In the display portion 9631 b, as in the display portion 9631 a, part ofthe display portion 9631 b can be a touch panel region 9632 b. When afinger, a stylus, or the like touches the place where a button 9639 forswitching to keyboard display is displayed in the touch panel, keyboardbuttons can be displayed on the display portion 9631 b.

Touch input can be performed concurrently on the touch panel regions9632 a and 9632 b.

The display-mode switching button 9034 allows switching between aportrait mode and a landscape mode, and between monochrome display andcolor display, for example. With the power-saving-mode switching button9036 for switching to power-saving mode, the luminance of display can beoptimized in accordance with the amount of external light at the timewhen the tablet terminal is in use, which is detected with an opticalsensor incorporated in the tablet terminal. The tablet terminal mayinclude another detection device such as a sensor for detectingorientation (e.g., a gyroscope or an acceleration sensor) in addition tothe optical sensor.

Although the display portion 9631 a and the display portion 9631 b havethe same display area in FIG. 23A, an embodiment of the presentinvention is not limited to this example. The display portion 9631 a andthe display portion 9631 b may have different areas or different displayquality. For example, one of them may be a display panel that candisplay higher-definition images than the other.

FIG. 23B illustrates the tablet terminal folded, which includes thehousing 9630, a solar battery 9633, and a charge and discharge controlcircuit 9634. Note that FIG. 23B illustrates an example in which thecharge and discharge control circuit 9634 includes a battery 9635 and aDCDC converter 9636.

Since the tablet terminal can be foldable, the housing 9630 can beclosed when the tablet terminal is not in use. Thus, the displayportions 9631 a and 9631 b can be protected, whereby a tablet terminalwith high endurance and high reliability for long-term use can beprovided.

The tablet terminal illustrated in FIGS. 23A and 23B can also have afunction of displaying various kinds of data (e.g., a still image, amoving image, and a text image), a function of displaying a calendar, adate, the time, or the like on the display portion, a touch-inputfunction of operating or editing data displayed on the display portionby touch input, a function of controlling processing by various kinds ofsoftware (programs), and the like.

The solar battery 9633, which is attached to the surface of the tabletterminal, supplies electric power to a touch panel, a display portion,an image signal processor, and the like. Note that the solar battery9633 can be provided on one or both surfaces of the housing 9630, sothat the battery 9635 can be charged efficiently. When a lithium ionbattery is used as the battery 9635, there is an advantage of downsizingor the like.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 23B are described with reference to a blockdiagram of FIG. 23C. The solar battery 9633, the battery 9635, the DCDCconverter 9636, a converter 9637, switches SW1, SW2, and SW3, and thedisplay portion 9631 are illustrated in FIG. 23C, and the battery 9635,the DCDC converter 9636, the converter 9637, and the switches SW1 to SW3correspond to the charge and discharge control circuit 9634 illustratedin FIG. 23B.

First, an example of operation in the case where power is generated bythe solar battery 9633 using external light is described. The voltage ofpower generated by the solar battery 9633 is raised or lowered by theDCDC converter 9636 so that a voltage for charging the battery 9635 isobtained. When the display portion 9631 is operated with the power fromthe solar battery 9633, the switch SW1 is turned on and the voltage ofthe power is raised or lowered by the converter 9637 to a voltage neededfor operating the display portion 9631. In addition, when display on thedisplay portion 9631 is not performed, the switch SW1 is turned off andthe switch SW2 is turned on so that charge of the battery 9635 may beperformed.

Here, the solar battery 9633 is illustrated as an example of a powergeneration means; however, there is no particular limitation on a way ofcharging the battery 9635, and the battery 9635 may be charged withanother power generation means such as a piezoelectric element or athermoelectric conversion element (Peltier element). For example, thebattery 9635 may be charged with a non-contact power transmission modulewhich is capable of charging by transmitting and receiving power bywireless (without contact), or another charging means may be used incombination.

Note that the structure and the like described in this embodiment can beused as appropriate in combination with any of the structures and thelike in the other embodiments.

Example 1

In this example, the resistances of an oxide semiconductor film and amultilayer film are described with reference to FIGS. 24A to 24D andFIG. 25.

First, the structure of a sample is described with reference to FIGS.24A to 24D.

FIG. 24A is a top view of a sample 1, a sample 2, a sample 3, and asample 4, and FIGS. 24B to 24D are cross-sectional views taken alongdashed-and-dotted line A1-A2 of FIG. 24A. Note that the top views of thesamples 1 to 4 are the same, and the cross-sectional views thereof aredifferent because the stacked-layer structures of the cross sections aredifferent. The cross-sectional views of the sample 1, the sample 2, andthe samples 3 and 4 are illustrated in FIG. 24B, FIG. 24C, and FIG. 24D,respectively.

As for the sample 1, an insulating film 1903 is formed over a glasssubstrate 1901, an insulating film 1904 is formed over the insulatingfilm 1903, and an oxide semiconductor film 1905 is formed over theinsulating film 1904. The both ends of the oxide semiconductor film 1905are covered with a conductive film 1907 and a conductive film 1909 eachserving as an electrode, and the oxide semiconductor film 1905 and theconductive films 1907 and 1909 are covered with an insulating film 1910and an insulating film 1911. Note that openings 1913 and 1915 areprovided in the insulating films 1910 and 1911, and the conductive film1907 and the conductive film 1909 are exposed through the opening 1913and the opening 1915, respectively.

As for the sample 2, the insulating film 1903 is formed over the glasssubstrate 1901, the insulating film 1904 is formed over the insulatingfilm 1903, and the oxide semiconductor film 1905 is formed over theinsulating film 1904. The both ends of the oxide semiconductor film 1905are covered with the conductive films 1907 and 1909 each serving as anelectrode, and the oxide semiconductor film 1905 and the conductivefilms 1907 and 1909 are covered with the insulating film 1911. Note thatan opening 1917 and an opening 1919 are provided in the insulating film1911, and the conductive film 1907 and the conductive film 1909 areexposed through the opening 1917 and the opening 1919, respectively.

In each of the samples 3 and 4, the insulating film 1903 is formed overthe glass substrate 1901, the insulating film 1904 is formed over theinsulating film 1903, and a multilayer film 1906 is formed over theinsulating film 1904. The both ends of the multilayer film 1906 arecovered with the conductive films 1907 and 1909 each serving as anelectrode, and the multilayer film 1906 and the conductive films 1907and 1909 are covered with the insulating film 1911. Note that theopenings 1917 and 1919 are provided in the insulating film 1911, and theconductive film 1907 and the conductive film 1909 are exposed throughthe opening 1917 and the opening 1919, respectively.

As described above, the structures of the insulating films in contactwith the top surface of the oxide semiconductor film 1905 or themultilayer film 1906 are different in the samples 1 to 4. In the sample1, the oxide semiconductor film 1905 and the insulating film 1910 are incontact with each other; in the sample 2, the oxide semiconductor film1905 and the insulating film 1911 are in contact with each other; and inthe samples 3 and 4, the multilayer film 1906 and the insulating film1911 are in contact with each other.

Next, fabrication methods of the samples are described.

First, a fabrication method of the sample 1 is described.

A 400-nm-thick silicon nitride film was formed as the insulating film1903 over the glass substrate 1901 by a plasma CVD method.

Next, a 50-nm-thick silicon oxynitride film was formed as the insulatingfilm 1904 over the insulating film 1903 by a plasma CVD method.

Next, a 35-nm-thick IGZO film was formed as the oxide semiconductor film1905 over the insulating film 1904 by a sputtering method using a metaloxide target containing In, Ga, and Zn at an atomic ratio of 1:1:1.Then, etching treatment was performed on the IGZO film with a maskformed through a photolithography process, so that the oxidesemiconductor film 1905 was formed.

Next, the conductive films 1907 and 1909 were formed over the insulatingfilm 1903 and the oxide semiconductor film 1905 in such a manner that a50-nm-thick tungsten film, a 400-nm-thick aluminum film, and a100-nm-thick titanium film were stacked in this order by a sputteringmethod, and were then subjected to etching treatment with a mask formedthrough a photolithography process.

Next, a 450-nm-thick silicon oxynitride film was formed as theinsulating film 1910 over the insulating film 1904, the oxidesemiconductor film 1905, the conductive film 1907, and the conductivefilm 1909 by a plasma CVD method, and then heat treatment was performedat 350° C. under a mixed atmosphere of nitrogen and oxygen for one hour.

Next, a 50-nm-thick silicon nitride film was formed as the insulatingfilm 1911 over the insulating film 1910 by a plasma CVD method.

Next, a mask was formed over the insulating film 1911 through aphotolithography process and then etching treatment was performed on theinsulating film 1911, so that the openings 1913 and 1915 were formed inthe insulating films 1910 and 1911.

Through the above process, the sample 1 was fabricated.

Next, a fabrication method of the sample 2 is described.

A 450-nm-thick silicon oxynitride film was formed as the insulating film1910 over the insulating film 1903, the oxide semiconductor film 1905,the conductive film 1907, and the conductive film 1909 of the sample 1by a plasma CVD method, and then heat treatment was performed at 350° C.under a mixed atmosphere of nitrogen and oxygen for one hour. Afterthat, the insulating film 1910 was removed.

Next, a 50-nm-thick silicon nitride film was formed as the insulatingfilm 1911 over the insulating film 1904, the oxide semiconductor film1905, the conductive film 1907, and the conductive film 1909 by a plasmaCVD method.

Next, a mask was formed over the insulating film 1911 through aphotolithography process and then etching treatment was performed on theinsulating film 1911, so that the openings 1917 and 1919 were formed inthe insulating film 1911.

Through the above process, the sample 2 was fabricated.

Next, a fabrication method of the sample 3 is described.

As for the sample 3, the only difference with the sample 2 is that themultilayer film 1906 was used instead of the oxide semiconductor film1905 of the sample 2. The multilayer film 1906 was formed over theinsulating film 1904 in such a manner that a 10-nm-thick IGZO film witha metal oxide target containing In, Ga, and Zn at an atomic ratio of1:3:2, a 10-nm-thick IGZO film with a metal oxide target containing In,Ga, and Zn at an atomic ratio of 1:1:1, and then a 10-nm-thick IGZO filmwith a metal oxide target containing In, Ga, and Zn at an atomic ratioof 1:3:2 were successively formed by a sputtering method. Then, etchingtreatment was performed on the stacked IGZO films with a mask formedthrough a photolithography process, so that the multilayer film 1906 wasformed.

Through the above process, the sample 3 was fabricated.

Next, a fabrication method of the sample 4 is described.

As for the sample 4, the only difference with the sample 2 is that themultilayer film 1906 was used instead of the oxide semiconductor film1905 of the sample 2. The thickness of the IGZO film included in themultilayer film 1906 of the sample 4 is different from that of thesample 3. The multilayer film 1906 was formed over the insulating film1904 in such a manner that a 20-nm-thick IGZO film with a metal oxidetarget containing In, Ga, and Zn at an atomic ratio of 1:3:2, a15-nm-thick IGZO film with a metal oxide target containing In, Ga, andZn at an atomic ratio of 1:1:1, and then a 10-nm-thick IGZO film using ametal oxide target containing In, Ga, and Zn at an atomic ratio of 1:3:2were successively formed by a sputtering method. Then, etching treatmentwas performed on the stacked IGZO films with a mask formed through aphotolithography process, so that the multilayer film 1906 was formed.

Through the above process, the sample 4 was fabricated.

Next, the sheet resistance of the oxide semiconductor film 1905 providedin each of the samples 1 and 2 and the sheet resistance of themultilayer film 1906 provided in each of the samples 3 and 4 weremeasured. In the sample 1, a probe is made contact with the openings1913 and 1915 to measure the sheet resistance of the oxide semiconductorfilm 1905. In each of the samples 2 to 4, a probe is made contact withthe openings 1917 and 1919 to measure the sheet resistance of the oxidesemiconductor film 1905 or the multilayer film 1906. Note that in theoxide semiconductor film 1905 in each of the samples 1 and 2 and themultilayer film 1906 in each of the samples 3 and 4, the widths of theconductive films 1907 and 1909 facing each other were each 1 mm and thedistance between the conductive films 1907 and 1909 was 10 μm. Further,in each of the samples 1 to 4, the potential of the conductive film 1907was a ground potential, and 1 V was applied to the conductive film 1909.

FIG. 25 shows the sheet resistance of the samples 1 to 4.

The sheet resistance of the sample 1 was about 1×10¹¹ Ω/s.q. The sheetresistance of the sample 2 was about 2620 Ω/s.q. The sheet resistance ofthe sample 3 was 4410 Ω/s.q. The sheet resistance of the sample 4 was2930 Ω/s.q.

In the above manner, the oxide semiconductor films 1905 and themultilayer films 1906 have different values of sheet resistance becausethe insulating films in contact with the oxide semiconductor film 1905and the insulating films in contact with the multilayer film 1906 weredifferent.

Note that when the above sheet resistances of the samples 1 to 4 wereconverted into resistivity, the resistivities of the sample 1, thesample 2, the sample 3, and the sample 4 were 3.9×10⁵ Ωcm, 9.3×10⁻³ Ωcm,1.3×10⁻² Ωcm, and 1.3×10⁻² Ωcm, respectively.

In the sample 1, the silicon oxynitride film used as the insulating film1910 was formed in contact with the top surface of the oxidesemiconductor film 1905 and apart from the silicon nitride film used asthe insulating film 1911. On the other hand, the silicon nitride filmused as the insulating film 1911 was formed in contact with the topsurface of the oxide semiconductor film 1905 in the sample 2 and wasformed in contact with the top surface of the multilayer film 1906 ineach of the samples 3 and 4. When the oxide semiconductor film 1905 orthe multilayer film 1906 is thus provided in contact with the siliconnitride film used as the insulating film 1911, defects, typically oxygenvacancies are generated in the oxide semiconductor film 1905 or themultilayer film 1906, and hydrogen contained in the silicon nitride filmis transferred to or diffused into the oxide semiconductor film 1905 orthe multilayer film 1906. Accordingly, the conductivity of the oxidesemiconductor film 1905 or the multilayer film 1906 is improved.

For example, in the case where an oxide semiconductor film is used for achannel formation region of a transistor, it is preferable to employ astructure in which a silicon oxynitride film is provided in contact withthe oxide semiconductor film as shown in the sample 1. Further, as alight-transmitting conductive film used for an electrode of a capacitor,it is preferable to employ a structure in which a silicon nitride filmis provided in contact with an oxide semiconductor film or a multilayerfilm as shown in the samples 2 to 4. With such a structure, even when anoxide semiconductor film or a multilayer film which is used for achannel formation region of a transistor and an oxide semiconductor filmor a multilayer film which is used for an electrode of a capacitor areformed through the same process, the resistivity of the oxidesemiconductor film and the resistivity of the multilayer film can bemade different from each other.

Next, the sheet resistance values of the samples 2 and 3 which werepreserved under a high-temperature high-humidity environment weremeasured. The conditions of the samples used here are described below.Note that here, the conditions are partly different from those of thesamples 2 and 3. Therefore, samples which have the same structure as thesamples 2 and 3 and which were formed under the different formationconditions are referred to as a sample 2a and a sample 3a.

First, a fabrication method of the sample 2a is described.

The insulating film 1903 and the insulating film 1904 were formed overthe glass substrate 1901.

Next, a 35-nm-thick IGZO film was formed as the oxide semiconductor film1905 over the insulating film 1904 by a sputtering method using a metaloxide target containing In, Ga, and Zn at an atomic ratio of 1:1:1.Then, etching treatment was performed on the IGZO film with a maskformed through a photolithography process and then heat treatment wasperformed at 350° C. or 450° C., so that the oxide semiconductor film1905 was formed.

Next, the conductive film 1907 and the conductive film 1909 were formedover the insulating film 1903 and the oxide semiconductor film 1905 insuch a manner that a 50-nm-thick titanium film and a 400-nm-thick copperfilm were stacked in this order by a sputtering method, and were thensubjected to etching treatment with a mask formed through aphotolithography process.

Next, a 450-nm-thick silicon oxynitride film was formed as theinsulating film 1910 over the insulating film 1904, the oxidesemiconductor film 1905, the conductive film 1907, and the conductivefilm 1909 by a plasma CVD method, and then heat treatment was performedat 350° C. under a mixed atmosphere of nitrogen and oxygen for one hour.

Next, a 50-nm-thick silicon nitride film was formed as the insulatingfilm 1911 over the insulating film 1904, the oxide semiconductor film1905, the conductive film 1907, and the conductive film 1909 by a plasmaCVD method. Note that the film formation temperature of the siliconnitride film was 220° C. or 350° C.

Next, a mask is formed over the insulating film 1911 through aphotolithography process and then etching treatment was performed on theinsulating film 1911, so that the opening 1913 and 1915 were formed inthe insulating films 1910 and 1911.

Through the above process, the sample 2a was fabricated.

Next, a fabrication method of the sample 3a is described.

As for the sample 3a, the only difference with the sample 2 is that themultilayer film 1906 was used instead of the oxide semiconductor film1905 of the sample 2a. The multilayer film 1906 was formed over theinsulating film 1904 in such a manner that a 10-nm-thick IGZO film witha metal oxide target containing In, Ga, and Zn at an atomic ratio of1:1:1, and a 10-nm-thick IGZO film with a metal oxide target containingIn, Ga, and Zn at an atomic ratio of 1:3:2 were successively formed by asputtering method. Then, etching treatment was performed on the stackedIGZO films with a mask formed through a photolithography process andthen heat treatment was performed at 350° C. or 450° C., so that themultilayer film 1906 was formed.

Through the above process, the sample 3a was fabricated.

Next, the sheet resistance of the oxide semiconductor film 1905 providedin the sample 2a and the sheet resistance of the multilayer film 1906provided in the sample 3a were measured. In each of the samples 2a and3a, a probe is made contact with the openings 1917 and 1919 to measurethe sheet resistance of the oxide semiconductor film 1905 or themultilayer film 1906. Note that in the oxide semiconductor film 1905 inthe sample 2a and the multilayer film 1906 in the sample 3a, the widthsof the conductive films 1907 and 1909 facing each other were each 1.5 mmand the distance between the conductive films 1907 and 1909 was 10 μm.Further, in each of the samples 2a and 3a, the potential of theconductive film 1907 was a ground potential, and 1 V was applied to theconductive film 1909. The sheet resistance values of the samples 2a and3a were measured after the samples 2a and 3a were preserved at 60° C.under an atmosphere with a humidity of 95% for 60 hours and 130 hours.

FIG. 29 shows the sheet resistance values of the samples 2a and 3a. Notethat in FIG. 29, the film formation temperature of the silicon nitridefilm formed as the insulating film 1910 in each sample is 220° C. (asolid line) or 350° C. (a dashed line). In addition, black circle andtriangle indicate the samples each subjected to heat treatment at 350°C. after the formation of the oxide semiconductor film 1905 or themultilayer film 1906, and white circle and triangle indicate the sampleseach subjected to heat treatment at 450° C. after the formation of theoxide semiconductor film 1905 or the multilayer film 1906. The circlesindicate the samples including the oxide semiconductor film 1905, i.e.the sample 2a. The triangles indicate the samples including themultilayer film 1906, i.e. the sample 3a. Still, measurements resultscorresponding to the multilayer film 1906 subjected to a 350° C. thermaltreatment, i.e. the sample 3a, are not plotted in the graph of FIG. 29.

FIG. 29 shows that the samples 2a and 3a had low sheet resistance valuesand satisfied a preferable sheet resistance value for an electrode of acapacitor, which is 0.2 Ω/s.q., and that the amount of change over timein the sheet resistance values of the samples 2a and 3a was small. Asdescribed above, the amount of change in the sheet resistance value ofthe oxide semiconductor film or the multilayer film in contact with thesilicon nitride film is small under a high-temperature high-humidityenvironment; therefore, the oxide semiconductor film or the multilayerfilm can be used as a light-transmitting conductive film which is usedfor an electrode of a capacitor.

Next, the sheet resistance values of the samples 2a and 3a when thesubstrate temperature was 25° C., 60° C., or 150° C. were measured, andthe measurement results are shown in FIG. 30. Note that here, as each ofthe samples 2a and 3a, a sample which includes the silicon nitride filmformed as the insulating film 1910 at 220° C. and which was subjected toheat treatment at 350° C. after the formation of the oxide semiconductorfilm 1905 or the multilayer film 1906 was used. The black circlesindicate measurement results regarding the sample 2a while the blacktriangles indicate measurements results regarding the sample 3a.

FIG. 30 shows that the sheet resistance value of the oxide semiconductorfilm 1905 or the multilayer film 1906 was not changed even when thesubstrate temperature was raised. In other words, the oxidesemiconductor film or the multilayer film in contact with the siliconnitride film is a degenerated semiconductor. The amount of change in thesheet resistance value of the oxide semiconductor film or the multilayerfilm in contact with the silicon nitride film was small even when thesubstrate temperature was changed; therefore, the oxide semiconductorfilm or the multilayer film can be used as a light-transmittingconductive film which is used for an electrode of a capacitor.

Note that the structure described in this example can be used asappropriate in combination with any of the structures in the otherembodiments and examples.

Example 2

In this example, analysis of impurities in an oxide semiconductor filmand an insulating film formed over the oxide semiconductor film isdescribed with reference to FIGS. 26A and 26B.

In this example, two kinds of samples (hereinafter a sample 5 and asample 6) were formed as samples for impurity analysis.

First, a fabrication method of the sample 5 is described below.

As for the sample 5, an IGZO film was formed over a glass substrate anda silicon nitride film was formed thereover. After that, heat treatmentat 450° C. under a nitrogen atmosphere for one hour and then heattreatment at 450° C. under a mixed gas atmosphere of nitrogen and oxygen(the proportion of the nitrogen was 80%, and the proportion of theoxygen was 20%) for one hour were successively performed.

Note that as for the IGZO film, a 100-nm-thick IGZO film was formed by asputtering method using a metal oxide target containing In, Ga, and Znat an atomic ratio of 1:1:1 under the following conditions: the Ar gasflow rate was 100 sccm and the O₂ gas flow rate was 100 sccm (theproportion of the O₂ gas was 50%); the pressure was 0.6 Pa; the filmformation power was 5000 W; and the substrate temperature was 170° C.

In addition, as for the silicon nitride film, a 100-nm-thick siliconnitride film was formed by a plasma CVD method under the followingconditions: the SiH₄ gas flow rate was 50 sccm, the N₂ gas flow rate was5000 sccm, and the NH₃ gas flow rate was 100 sccm; the pressure was 100Pa; the film formation power was 1000 W; and the substrate temperaturewas 220° C.

Next, a fabrication method of the sample 6 is described below.

An IGZO film was formed over a glass substrate and a silicon oxynitridefilm and a silicon nitride film were stacked thereover. After that, heattreatment at 450° C. under a nitrogen atmosphere for one hour and thenheat treatment at 450° C. under a mixed gas atmosphere of nitrogen andoxygen (the proportion of the nitrogen was 80%, and the proportion ofthe oxygen was 20%) for one hour were successively performed.

Note that the film formation conditions of the IGZO film and the siliconnitride film were similar to those of the sample 5. In addition, as forthe silicon oxynitride film, a 50-nm-thick silicon oxynitride film wasformed by a plasma CVD method under the following conditions: the SiH₄gas flow rate was 30 sccm and the N₂O gas flow rate was 4000 sccm; thepressure was 40 Pa; the film formation power was 150 W; and thesubstrate temperature was 220° C. After that, a 400-nm-thick siliconoxynitride film was formed by a plasma CVD method under the followingconditions: the SiH₄ gas flow rate was 160 sccm and the N₂O gas flowrate was 4000 sccm; the pressure was 200 Pa; the film formation powerwas 1500 W; and the substrate temperature was 220° C.

FIGS. 26A and 26B show the results of the impurity analysis of thesamples 5 and 6.

Note that the impurity analysis was performed in the direction shown bythe arrow in each of FIGS. 26A and 26B by secondary ion massspectrometry (SIMS). That is, the measurement was performed from theglass substrate side.

FIG. 26A shows the concentration profile of hydrogen (H) which wasobtained by measurement of the sample 5. FIG. 26B shows theconcentration profile of hydrogen (H) which was obtained by measurementof the sample 6.

FIG. 26A shows that the concentration of hydrogen (H) in the IGZO filmwas 1.0×10²⁰ atoms/cm³ and that the concentration of hydrogen (H) in thesilicon nitride film was 1.0×10²³ atoms/cm³. In addition, FIG. 26B showsthat the concentration of hydrogen (H) in the IGZO film was 5.0×10¹⁹atoms/cm³ and that the concentration of hydrogen (H) in the siliconoxynitride film was 3.0×10²¹ atoms/cm³.

It is known that it is difficult to obtain accurate data in theproximity of a surface of a sample or in the proximity of an interfacebetween stacked films formed using different materials by the SIMSanalysis in measurement principle. Thus, in the case where distributionsof the concentrations of hydrogen (H) in the film in the thicknessdirection are analyzed by SIMS, an average value in a region where thefilm is provided, the value is not greatly changed, and an almostconstant level of strength can be obtained is employed as theconcentrations of hydrogen (H).

A difference between the IGZO films in the concentration of hydrogen (H)was found in this manner by changing the structure of the insulatingfilm in contact with the IGZO film.

For example, in the case where any of the above IGZO films is formed ina channel formation region of a transistor, it is preferable to employ astructure in which a silicon oxynitride film is provided in contact withthe IGZO film as shown in the sample 6. As a light-transmittingconductive film used for an electrode of a capacitor, it is preferableto employ a structure in which a silicon nitride film is provided incontact with the IGZO film as shown in the sample 5. With such astructure, even when an IGZO film which is used for a channel formationregion of a transistor and an IGZO film which is used for an electrodeof a capacitor are formed through the same process, the hydrogenconcentrations of the IGZO films can be made different from each other.

Example 3

In this example, the amounts of defects in an oxide semiconductor filmand a multilayer film are described with reference to FIGS. 27A to 27Cand FIG. 28.

First, the structure of a sample is described.

A sample 7 includes a 35-nm-thick oxide semiconductor film formed over aquartz substrate and a 100-nm-thick nitride insulating film formed overthe oxide semiconductor film.

A sample 8 and a sample 9 each include a 30-nm-thick multilayer filmformed over a quartz substrate and a 100-nm-thick nitride insulatingfilm formed over the multilayer film. Note that in the multilayer filmof the sample 8, a 10-nm-thick first IGZO film, a 10-nm-thick secondIGZO film, and a 10-nm-thick third IGZO film are stacked in this order.In the multilayer film of the sample 9, a 20-nm-thick first IGZO film, a15-nm-thick second IGZO film, and a 10-nm-thick third IGZO film arestacked in this order. The samples 8 and 9 are different from the sample7 in that the multilayer film is included instead of the oxidesemiconductor film.

A sample 10 includes a 100-nm-thick oxide semiconductor film formed overa quartz substrate, a 250-nm-thick oxide insulating film formed over theoxide semiconductor film, and a 100-nm-thick nitride insulating filmformed over the oxide insulating film. The sample 10 is different fromthe samples 7 to 9 in that the oxide semiconductor film is not incontact with the nitride insulating film but in contact with the oxideinsulating film.

Next, fabrication methods of the samples are described.

First, a fabrication method of the sample 7 is described.

A 35-nm-thick IGZO film was formed as the oxide semiconductor film overthe quartz substrate. As for the IGZO film, the 35-nm-thick IGZO filmwas formed by a sputtering method using a metal oxide target containingIn, Ga, and Zn at an atomic ratio of 1:1:1 under the followingconditions: the Ar gas flow rate was 100 sccm and the O₂ gas flow ratewas 100 sccm (the proportion of the O₂ gas was 50%); the pressure was0.6 Pa; the film formation power was 5000 W; and the substratetemperature was 170° C.

Next, as first heat treatment, heat treatment at 450° C. under anitrogen atmosphere for one hour and then heat treatment at 450° C.under a mixed gas atmosphere of nitrogen and oxygen (the proportion ofthe nitrogen was 80%, and the proportion of the oxygen was 20%) for onehour were successively performed.

Next, a 100-nm-thick silicon nitride film was formed as the nitrideinsulating film over the oxide semiconductor film. As for the siliconnitride film, the 100-nm-thick silicon nitride film was formed by aplasma CVD method under the following conditions: the SiH₄ gas flow ratewas 50 sccm, the N₂ gas flow rate was 5000 sccm, and the NH₃ gas flowrate was 100 sccm; the pressure was 100 Pa; the film formation power was1000 W; and the substrate temperature was 350° C.

Next, as second heat treatment, heat treatment was performed at 250° C.under a nitrogen atmosphere for one hour.

Through the above process, the sample 7 was fabricated.

Next, a fabrication method of the sample 8 is described.

As for the sample 8, the multilayer film was formed instead of the oxidesemiconductor film of the sample 7. As for the multilayer film, the10-nm-thick first IGZO film was formed by a sputtering method using ametal oxide target containing In, Ga, and Zn at an atomic ratio of 1:3:2under the following conditions: the Ar gas flow rate was 180 sccm andthe O₂ gas flow rate was 20 sccm (the proportion of the O₂ gas was 10%);the pressure was 0.6 Pa; the film formation power was 5000 W; and thesubstrate temperature was 25° C. Then, the 10-nm-thick second IGZO filmwas formed by a sputtering method using a metal oxide target containingIn, Ga, and Zn at an atomic ratio of 1:1:1 under the followingconditions: the Ar gas flow rate was 100 sccm and the O₂ gas flow ratewas 100 sccm (the proportion of the O₂ gas was 50%); the pressure was0.6 Pa; the film formation power was 5000 W; and the substratetemperature was 170° C. Then, the 10-nm-thick third IGZO film was formedby a sputtering method using a metal oxide target containing In, Ga, andZn at an atomic ratio of 1:3:2 under the following conditions: the Argas flow rate was 180 sccm and the 02 gas flow rate was 20 sccm (theproportion of the O₂ gas was 10%); the pressure was 0.6 Pa; the filmformation power was 5000 W; and the substrate temperature was 25° C.

Other steps are similar to those of the sample 7. Through the aboveprocess, the sample 8 was fabricated.

Next, a fabrication method of the sample 9 is described.

As for the sample 9, the multilayer film was formed instead of the oxidesemiconductor film of the sample 7. As for the multilayer film, the20-nm-thick first IGZO film was formed over the quartz substrate underthe same conditions as the first IGZO film of the sample 8. Then, the15-nm-thick second IGZO film was formed by a sputtering method under thesame conditions as the second IGZO film of the sample 8. Then, the10-nm-thick second IGZO film was formed under the same conditions as thethird IGZO film of the sample 8.

Other steps are similar to those of the sample 7. Through the aboveprocess, the sample 9 was fabricated.

Next, a fabrication method of the sample 10 is described.

As for the sample 10, the 100-nm-thick oxide semiconductor film wasformed over the quartz substrate under the same conditions as the sample7.

Next, first heat treatment was performed under conditions similar tothose of the sample 7.

Next, a 50-nm-thick first silicon oxynitride film and a 200-nm-thicksecond silicon oxynitride film were stacked over the oxide semiconductorfilm as the oxide insulating film. Here, the 50-nm-thick first siliconoxynitride film was formed by a plasma CVD method under the followingconditions: the SiH₄ gas flow rate was 30 sccm and the N₂O gas flow ratewas 4000 sccm; the pressure was 40 Pa; the film formation power was 150W; and the substrate temperature was 220° C. After that, the200-nm-thick second silicon oxynitride film was formed by a plasma CVDmethod under the following conditions: the SiH₄ gas flow rate was 160sccm and the N₂O gas flow rate was 4000 sccm; the pressure was 200 Pa;the film formation power was 1500 W; and the substrate temperature was220° C. Note that the second silicon oxynitride film is a filmcontaining oxygen at a higher proportion than oxygen in thestoichiometric composition.

Next, a 100-nm-thick silicon nitride film was formed over the oxideinsulating film under the same conditions as the sample 7.

Next, second heat treatment was performed under conditions similar tothose of the sample 7.

Through the above process, the sample 10 was fabricated.

Next, the samples 7 to 10 were measured by ESR. In the ESR measurementperformed at a predetermined temperature, a value of a magnetic field(H₀) where a microwave is absorbed is used for an equation g=hn/bH₀, sothat a parameter of a g-factor can be obtained. Note that the frequencyof the microwave is denoted by v, and the Planck constant and the Bohrmagneton are denoted by, respectively, h and 0 which are both constants.

Here, the ESR measurement was performed under the conditions as follows.The measurement temperature was room temperature (25° C.), thehigh-frequency power (power of microwaves) of 8.92 GHz was 20 mW, andthe direction of a magnetic field was parallel to a surface of eachsample.

FIG. 27A shows a first derivative curve obtained by ESR measurement ofthe oxide semiconductor film in the sample 7; and FIGS. 27B and 27C showfirst derivative curves obtained by ESR measurement of the multilayerfilms in the samples 8 and 9. FIG. 27A shows the measurement result ofthe sample 7, FIG. 27B shows the measurement result of the sample 8, andFIG. 27C shows the measurement result of the sample 9.

FIG. 28 shows a first derivative curve obtained by ESR measurement ofthe oxide semiconductor film in the sample 10.

In FIGS. 27A to 27C, the sample 7 has signal symmetry due to defectshaving a g-factor of 1.93 in the oxide semiconductor film. The samples 8and 9 each have signal symmetry due to defects having a g-factor of 1.95in the multilayer film. As for the sample 7, the spin densitycorresponding to a g-factor of 1.93 was 2.5×10¹⁹ spins/cm³, in thesample 8, the total spin densities corresponding to g-factors of 1.93and 1.95 were 1.6×10¹⁹ spins/cm³, and in the sample 9, the total spindensities corresponding to g-factors of 1.93 and 1.95 were 2.3×10¹⁹spins/cm³. That is, it is found that the oxide semiconductor film andthe multilayer film include defects. Note that an oxygen vacancy is anexample of the defect in the oxide semiconductor film and the multilayerfilm.

Although, in FIG. 28, the thickness of the oxide semiconductor film ofthe sample 10 is greater than that of the oxide semiconductor film ofthe sample 7 and those of the multilayer films of the samples 8 and 9,signal symmetry due to a defect was not detected, i.e. the number ofdefects was less than or equal to the lower limit of detection (here,the lower limit of detection was 3.7×10¹⁶ spins/cm³). Accordingly, it isfound that the number of defects in the oxide semiconductor film cannotbe detected.

It is found that when a nitride insulating film, here the siliconnitride film formed by a plasma CVD method is in contact with an oxidesemiconductor film or a multilayer film, defects, typically oxygenvacancies, are generated in the oxide semiconductor film or themultilayer film. On the other hand, when an oxide insulating film, herethe silicon oxynitride film, is provided on an oxide semiconductor film,excess oxygen contained in the silicon oxynitride film, i.e. oxygencontained at a higher proportion than oxygen in the stoichiometriccomposition is diffused into the oxide semiconductor film and thus thenumber of defects in the oxide semiconductor film is not increased.

As described above, as shown in the samples 7 to 9, the oxidesemiconductor film or the multilayer film which is in contact with thenitride insulating film has a number of defects, typically oxygenvacancies, and has a high conductivity and therefore can be used as anelectrode of a capacitor. On the other hand, as shown in the sample 10,an oxide semiconductor film or a multilayer film which is in contactwith the oxide insulating film has a small number of oxygen vacanciesand low conductivity and therefore can be used as a channel formationregion of a transistor.

Here, the cause of a reduction in resistivity of the oxide semiconductorfilm or the multilayer film which is in contact with the nitrideinsulating film is described below.

<Energy and Stability Between Existing Modes of Hydrogen (H)>

First, the energy difference and stability in a mode of H which existsin an oxide semiconductor film is described with calculated results.Here, InGaZnO₄ was used as the oxide semiconductor film.

The structure used for the calculation is based on a 84-atom bulk modelin which twice the number of a hexagonal unit cell of the InGaZnO₄ isarranged along the a-axis and b-axis.

As the bulk model, a model in which one 0 atom bonded to three In atomsand one Zn atom is substituted with a H atom was prepared (see FIG.31A). FIG. 31B shows a diagram in which the a-b plane of the InO layerin FIG. 31A is viewed from the c-axis direction. A region from which oneO atom bonded to three In atoms and one Zn atom is removed is shown asan oxygen vacancy Vo, which is shown in a dashed line in FIGS. 31A and31B. In addition, a H atom in the oxygen vacancy Vo is expressed as VoH.

In the bulk model, one O atom bonded to three In atoms and one Zn atomis removed, whereby an oxygen vacancy Vo is formed. A model in which, inthe vicinity of the oxygen vacancy Vo, a H atom is bonded to one O atomto which one Ga atom and two Zn atoms are bonded on the a-b plane wasprepared (see FIG. 31C). FIG. 31D shows a diagram in which the a-b planeof the InO layer in FIG. 31C is viewed from the c-axis direction. InFIGS. 31C and 31D, an oxygen vacancy Vo is shown in a dashed line. Amodel in which an oxygen vacancy Vo is formed and, in the vicinity ofthe oxygen vacancy Vo, a H atom is bonded to one O atom to which one Gaatom and two Zn atoms are bonded on the a-b plane is expressed as Vo+H.

Optimization calculation was performed on the above two models with afixed lattice constant to calculate the total energy. Note that as thevalue of the total energy is smaller, the structure becomes more stable.

In the calculation, first principles calculation software VASP (TheVienna Ab initio Simulation Package) was used. The calculationconditions are shown in Table 1.

TABLE 1 Software VASP Pseudopotential PAW Functional CGA/PBE Cut-offenergy 500 eV K-point 4 × 4 × 1

As pseudopotential calculation of electronic states, a potentialgenerated by a projector augmented wave (PAW) method was used, and as afunctional, generalized-gradient-approximation/Perdew-Burke-Ernzerhof(GGA/PBE) was used.

In addition, the total energy of the two models which were obtained bythe calculations is shown in Table 2.

TABLE 2 Model Total Energy VoH −456.084 eV Vo + H −455.304 eV

According to Table 2, the total energy of VoH is lower than that of Vo+Hby 0.78 eV. Thus, VoH is more stable than Vo+H. Accordingly, when a Hatom comes close to an oxygen vacancy (Vo), the H atom might be easilytrapped in the oxygen vacancy (Vo) than bonding with an O atom.

<Thermodynamic State of VoH>

Next, the formation energy and the charge state of VoH which isgenerated by a H atom trapped in an oxygen vacancy (Vo) is describedwith calculated results. The formation energy of VoH is differentdepending on the charge state and also depends on the Fermi energy.Thus, the stable charge state of VoH is different depending on the Fermienergy. Here, (VoH)⁺ denotes a state in which one electron is dischargedby VoH, (VoH)⁻ denotes a state in which one electron is trapped by VoH,and (VoH)⁰ denotes a state in which an electron is not transferred. Theformation energies of (VoH)⁺, (VoH)⁻, and (VoH)⁰ were calculated.

In the calculation, the first principles calculation software VASP wasused. The calculation conditions are shown in Table 3.

TABLE 3 Software VASP Pseudopotential PAW Functional HSE06 Cut-offenergy 800 eV Number of k -point sampling 2 × 2 × 1 (opt.) 4 × 4 × 1(single) Spin polarization setup Shielding parameter 0.2 Fraction of thenonolcal Fock-exchange 0.25 Number of atoms 84

As pseudopotential calculation of electronic states, a potentialgenerated by a projector augmented wave (PAW) method was used, and as afunctional, Heyd-Scuseria-Ernzerhof (HSE) DFT hybrid factor (HSE06) wasused.

Note that the formation energy of an oxygen vacancy was calculated asfollows: a dilute limit of the concentration of oxygen vacancies wasassumed, and excessive expansion of electrons and holes to theconduction band and the valence band was corrected. In addition, shiftof the valence band due to the defect structure was corrected using theaverage electrostatic potential with the top of the valence band of acomplete crystal serving as the origin of energy.

FIG. 32A shows the formation energies of (VoH)⁺, (VoH)⁻, and (VoH)⁰. Thehorizontal axis represents the Fermi level, and the vertical axisrepresents the formation energy. The solid line represents the formationenergy of (VoH)⁺, the dashed-dotted line represents the formation energyof (VoH)⁰, and the dashed line represents the formation energy of(VoH)⁻. In addition, the transition level of the VoH charge from (VoH)⁺to (VoH)⁻ through (VoH)⁰ is represented by ∈ (+/−).

FIG. 32B shows a thermodynamic transition level of VoH. From thecalculation result, the energy gap of InGaZnO₄ was 2.739 eV. Inaddition, when the energy of the valence band is 0 eV, the transferlevel (∈ (+/−)) is 2.62 eV, which exists just under the conduction band.This shows that InGaZnO₄ is n-type by trapping a H atom in an oxygenvacancy Vo.

When an oxide semiconductor film is exposed to plasma, the oxidesemiconductor film is damaged and defects, typically oxygen vacanciesare generated in the oxide semiconductor film. In addition, when anitride insulating film is in contact with an oxide semiconductor film,hydrogen contained in the nitride insulating film is transferred to theoxide semiconductor film. As a result, VoH is formed in an oxidesemiconductor film by entry of hydrogen into an oxygen vacancy in theoxide semiconductor film, so that the oxide semiconductor film becomesn-type film and the resistivity thereof is reduced. As described above,the oxide semiconductor film in contact with the nitride insulating filmcan be used as an electrode of a capacitor.

This application is based on Japanese Patent Application serial No.2012-192214 filed with Japan Patent Office on Aug. 31, 2012 and JapanesePatent Application serial No. 2013-054021 filed with Japan Patent Officeon Mar. 15, 2013, the entire contents of which are hereby incorporatedby reference.

What is claimed is:
 1. A semiconductor device comprising: a substrate; afirst insulating film over the substrate; a second insulating film overthe first insulating film; a third insulating film over the secondinsulating film; a fourth insulating film over the third insulatingfilm, the fourth insulating film comprising carbon; a light-transmittingconductive film over the fourth insulating film; a transistorcomprising: a gate electrode; the first insulating film over the gateelectrode; and a semiconductor film over the first insulating film andoverlapping the gate electrode, the semiconductor film beingelectrically connected to the light-transmitting conductive film; and acapacitor comprising: a light-transmitting film as a first capacitorelectrode over the first insulating film; the third insulating film andthe fourth insulating film as at least part of a capacitor dielectricfilm over the light-transmitting film and the light-transmittingconductive film as a second capacitor electrode over the fourthinsulating film, wherein the second insulating film and the thirdinsulating film cover the semiconductor film, wherein the secondinsulating film is in direct contact with a first portion of thelight-transmitting film, and wherein the third insulating film is indirect contact with a second portion of the light-transmitting film. 2.The semiconductor device according to claim 1, wherein the firstinsulating film comprises a first oxide insulating film that is formedby a chemical vapor deposition method using a deposition gas containingsilicon and an oxidizing gas, and wherein the fourth insulating film isa second oxide insulating film that is formed by a chemical vapordeposition method using an organosilane gas.
 3. The semiconductor deviceaccording to claim 1, wherein the first insulating film is partly etchedin a region under the light-transmitting film.
 4. The semiconductordevice according to claim 1, wherein the first capacitor electrode andthe semiconductor film are formed from a same film.
 5. The semiconductordevice according to claim 1, wherein the first capacitor electrode andthe semiconductor film are formed from an oxide semiconductor film. 6.The semiconductor device according to claim 1, wherein the firstcapacitor electrode and the semiconductor film are formed from an oxidesemiconductor film, and wherein the first capacitor electrode has ahigher conductivity than the semiconductor film.
 7. The semiconductordevice according to claim 1, further comprising a capacitor line,wherein the light-transmitting film contains a semiconductor material,and wherein the semiconductor device is configured so that a potentialapplied to the capacitor line is constantly lower than a potential to besupplied to the light-transmitting conductive film by a thresholdvoltage of the capacitor or more when the semiconductor device is used.8. A display device comprising the semiconductor device according toclaim
 1. 9. An electronic device comprising the semiconductor deviceaccording to claim
 1. 10. The semiconductor device according to claim 1,wherein the first portion does not overlaps with the second portion. 11.A semiconductor device comprising: a substrate; a first insulating filmover the substrate, the first insulating film being a first oxideinsulating film; a second insulating film over the first insulatingfilm, the second insulating film being a second oxide insulating film; athird insulating film over the second insulating film, the thirdinsulating film being a nitride insulating film; a fourth insulatingfilm over the third insulating film, the fourth insulating film being athird oxide insulating film, and the fourth insulating film comprisingcarbon; a light-transmitting conductive film over the fourth insulatingfilm; a transistor comprising: a gate electrode; the first insulatingfilm over the gate electrode; and a metal oxide semiconductor film overthe first insulating film and overlapping the gate electrode, the metaloxide semiconductor film being electrically connected to thelight-transmitting conductive film; and a capacitor comprising: alight-transmitting film as a first capacitor electrode over the firstinsulating film; the third insulating film and the fourth insulatingfilm as at least part of a capacitor dielectric film over thelight-transmitting film; and the light-transmitting conductive film as asecond capacitor electrode over the fourth insulating film, wherein thefirst insulating film and the second insulating film are each in directcontact with the metal oxide semiconductor film, wherein the secondinsulating film and the third insulating film cover the metal oxidesemiconductor film, wherein the second insulating film is in directcontact with a first portion of the light-transmitting film, and whereinthe third insulating film is in direct contact with a second portion ofthe light-transmitting film.
 12. The semiconductor device according toclaim 11, wherein the first insulating film is formed by a chemicalvapor deposition method using a deposition gas containing silicon and anoxidizing gas, and wherein the fourth insulating film is formed by achemical vapor deposition method using an organosilane gas.
 13. Thesemiconductor device according to claim 11, wherein the first insulatingfilm comprises a nitride insulating film under the first oxideinsulating film, and wherein the light-transmitting film is in contactwith the nitride insulating film.
 14. The semiconductor device accordingto claim 11, wherein the light-transmitting film and the metal oxidesemiconductor film are formed from an oxide semiconductor film, andwherein the light-transmitting film further contains a dopant at aconcentration greater than 1×10¹⁹ atoms/cm³ and less than or equal to1×10²² atoms/cm³.
 15. The semiconductor device according to claim 11,wherein the light-transmitting film and the metal oxide semiconductorfilm are formed from an oxide semiconductor film, and wherein thelight-transmitting film contains a higher concentration of nitrogen thanthe metal oxide semiconductor film.
 16. The semiconductor deviceaccording to claim 11, wherein the light-transmitting film and the metaloxide semiconductor film are formed from an oxide semiconductor film,and wherein the light-transmitting film contains a higher concentrationof hydrogen than the metal oxide semiconductor film.
 17. Thesemiconductor device according to claim 11, further comprising acapacitor line, wherein the light-transmitting film contains asemiconductor material, and wherein the semiconductor device isconfigured so that a potential applied to the capacitor line isconstantly lower than a potential to be supplied to thelight-transmitting conductive film by a threshold voltage of thecapacitor or more when the semiconductor device is used.
 18. A displaydevice comprising the semiconductor device according to claim
 11. 19. Anelectronic device comprising the semiconductor device according to claim11.
 20. The semiconductor device according to claim 11, wherein thefirst portion does not overlaps with the second portion.
 21. Thesemiconductor device according to claim 11, wherein the metal oxidesemiconductor film and the light-transmitting film are formed from asame film.
 22. A semiconductor device comprising: a substrate; a firstinsulating film over the substrate; a second insulating film over thefirst insulating film; a third insulating film over the secondinsulating film, the third insulating film being a nitride insulatingfilm comprising silicon; a fourth insulating film over the thirdinsulating film, the fourth insulating film being a oxide insulatingfilm comprising silicon and carbon; a light-transmitting conductive filmover the fourth insulating film; a transistor comprising: a gateelectrode; the first insulating film over the gate electrode; and ametal oxide semiconductor film over the first insulating film andoverlapping the gate electrode, the metal oxide semiconductor film beingelectrically connected to the light-transmitting conductive film; acapacitor comprising: a light-transmitting film as a first capacitorelectrode over the first insulating film; the third insulating film andthe fourth insulating film as at least part of a capacitor dielectricfilm over the light-transmitting film; and the light-transmittingconductive film as a second capacitor electrode over the fourthinsulating film, wherein the second insulating film and the thirdinsulating film cover the metal oxide semiconductor film, wherein thesecond insulating film is in direct contact with a first portion of thelight-transmitting film, and wherein the third insulating film is indirect contact with a second portion of the light-transmitting film. 23.The semiconductor device according to claim 22, wherein the firstinsulating film is formed by a chemical vapor deposition method using adeposition gas containing silicon and an oxidizing gas, and wherein thefourth insulating film is formed by a chemical vapor deposition methodusing an organosilane gas.
 24. The semiconductor device according toclaim 22, wherein the first insulating film comprises a nitrideinsulating film and an oxide insulating film over the nitride insulatingfilm, and wherein the light-transmitting film is in contact with thenitride insulating film.
 25. The semiconductor device according to claim22, wherein the light-transmitting film and the metal oxidesemiconductor film are formed from an oxide semiconductor film, andwherein the light-transmitting film further contains a dopant at aconcentration greater than 1×10¹⁹ atoms/cm³ and less than or equal to1×10²² atoms/cm³.
 26. The semiconductor device according to claim 22,wherein the light-transmitting film and the metal oxide semiconductorfilm are formed from an oxide semiconductor film, and wherein thelight-transmitting film contains a higher concentration of nitrogen thanthe metal oxide semiconductor film.
 27. The semiconductor deviceaccording to claim 22, wherein the light-transmitting film and the metaloxide semiconductor film are formed from an oxide semiconductor film,and wherein the light-transmitting film contains a higher concentrationof hydrogen than the metal oxide semiconductor film.
 28. Thesemiconductor device according to claim 22, further comprising acapacitor line, wherein the light-transmitting film contains asemiconductor material, and wherein the semiconductor device isconfigured so that a potential applied to the capacitor line isconstantly lower than a potential to be supplied to thelight-transmitting conductive film by a threshold voltage of thecapacitor or more when the semiconductor device is used.
 29. A displaydevice comprising the semiconductor device according to claim
 21. 30. Anelectronic device comprising the semiconductor device according to claim22.
 31. The semiconductor device according to claim 22, wherein thefirst portion does not overlaps with the second portion.
 32. Thesemiconductor device according to claim 22, wherein the metal oxidesemiconductor film and the light-transmitting film are formed from asame film.